Methods of expressing proteins in insect cells and methods of killing insects

ABSTRACT

Described herein is a method of expressing heterologous proteins in insect cells using an expression cassette comprising a structural gene for a heterologous protein physically attached to an insect cellular promoter and an enhancer. The cells may also express the IE-1 product. Also described herein is a method of killing insects comprising infecting the insects with a recombinant baculovirus comprising a structural gene for an incompatible protein functionally linked to an insect cellular promoter and an enhancer. The invention is also directed towards expression cassettes comprising an insect cellular promoter functionally linked to an enhancer wherein the promoter is capable of directing the expression of a heterologous protein in tissues containing the expression cassette, recombinant expression cassettes containing heterologous proteins, transplacement fragments, vectors and recombinant baculoviruses.

This application is a continuation of application Ser. No. 08/931,830,filed Sep. 16, 1997, now U.S. Pat. No. 5,989,541, which is a divisionalof U.S. Ser. No. 08/608,617 filed on Mar. 1, 1996 (now U.S. Pat. No.5,759,809) which in turn is a continuation of U.S. Ser. No. 08/172,653filed on Dec. 23, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of expressing heterologousproteins in insect cells using genetic elements which potentiateactivity of an insect cellular promoter functionally attached to astructural gene for a heterologous protein. The heterologous protein mayinclude proteins toxic or harmful to an insect host or proteins whoseunregulated expression will incapacitate the insect host. The inventionis also directed to expression cassettes, recombinant expressioncassettes containing heterologous genes, transplacement fragmentscontaining expression cassettes, transplacement fragments containingrecombinant expression cassettes, vectors containing tansplacementfragments and recombinant baculoviruses derived therefrom.

2. State of the Art

Nuclear polyhedrosis viruses (NPVs) are a subgroup of the familyBaculoviridae, whose virions are embedded into proteinaceous polyhedrain the nucleus of host cells. Baculoviruses provide alternatives tochemicals for controlling insect pests. Because most NPVs have a hostrange restricted to only a few closely related species, they can be usedwithout disrupting the balance of other insect and non-insect species(e.g. important predators) in the agricultural ecosystem. No baculovirushas been demonstrated to infect mammals, reptiles, birds, invertebratessuch as earthworms or plants. To date, however, baculoviruses have metwith only limited commercial success as control agents, due todifficulties with virus stability and, most importantly, slower speed ofaction than that achieved with chemical insecticides.

Certain baculoviruses, specifically nuclear polyhedrosis viruses (NPVs),have a unique life cycle which involves the temporally regulatedexpression of two functionally and morphologically different viralforms, the budded form and the occluded form. Nuclear polyhedrosisviruses produce large polyhedral occlusion bodies, which containenveloped virus particles, within the nucleus of infected cells. Theocclusion body is composed of a matrix comprising a 29 kDa protein knownas polyhedron. After the insect dies from infection, occlusion bodiescontaining virus are released from the dead larvae into the environmentand spread the infection to other insects through contamination of thefood supply. These occlusion bodies serve to protect the virus particlesin the environment and also provide a means of delivering the virusparticles to the primary site of infection in insects, the midgutepithelial cells. When the occlusion bodies are ingested by the larvae,the alkaline pH of the midgut lumen of phytophagous lepidopteran larvaedissolves the paracrystalline matrix in which the virus particles areembedded, promoting infection.

Secondary infection within the insect involves the budded form of thevirus. Viral nucleocapsids are synthesized in the nucleus of the insectcell, move through the cytoplasm and bud from the plasma membrane of thecell resulting in the release of budded virus particles into the insecthemolymph. The open circulatory system of the insect provides the viruswith access to other tissues of the insect. Virtually all tissues withinthe host larvae are susceptible to infection by the budded virus.Replication of the virus in other organs creates extensive tissue damageand eventually death. Generally, the complete process can take 4-5 daysin the laboratory, but may take more than a week in the field.

The synthesis of the budded and occluded forms of the virus istemporally regulated. During a typical infection of host tissue culturecells, progeny budded viruses are released into the culture mediabeginning approximately 12 hour post infection (p.i.) and the releasecontinues logarithmically through 22 hours p.i. Occluded virus formsapproximately at 20 hours p.i. and continues through 70 hours p.i. bywhich time approximately 70-100 polyhedral occlusions have formed in thenucleus. This temporal regulation of viral development is reflected inthe controlled transcription of specific viral genes.

Nuclear polyhedrosis virus genes are transcribed in a regulated cascadeinvolving at least three phases of transcription: an early phase (0-6hours p.i.) prior to viral DNA replication, a late phase (6-18 hoursp.i.) involving DNA replication and budded virus formation and the verylate occlusion phase (18 through 70 hours p.i.).

In contrast to chemical pesticides which usually act upon the targetinsect immediately upon application, baculovirus infection-mediatedreduction of the population of insects occurs in the field only one totwo weeks after application of wild-type baculovirus. In order toincrease the speed of insect inactivation by baculoviruses, recombinantviruses have been generated which express non-viral proteins whoseproducts are toxic to the infected insect, under the control of viral orsynthetic promoters. The underlying premise for the creation of suchrecombinant viruses has been that the expression of the foreign proteinin the larvae should inactivate or kill the larvae before they wouldnormally succumb to viral infection. Some recombinant viruses have beendeveloped which employ the viral polyhedron promoter (Merryweather etal., 1990; Tomalski and Miller, 1991; Maeda et al., 1991), the viral p10promoter (Stewart et al., 1991; McCutchen et al., 1991), or a syntheticpromoter based on the previous two (Wang et al., 1991) to express thedesired foreign protein. While these viral promoters can direct theexpression of high levels of protein, they are not expressed until thevery late occlusion stage of infection.

One of the key aspects of the development of recombinant baculovirusesas effective insecticides is the timing and site of expression ofheterologous proteins following initial infection of the target insect.When larvae are infected orally with relatively low doses of polyhedra,as would normally occur under field conditions, the first cells to beinfected are the columnar and regenerative cells of the midgutepithelium. The generalized spread of the virus to other tissues of aninfected larva through circulation does not occur until 36 hours afterthe virus is first observed in the gut epithelium. Expression ofheterologous genes under the control of the polyhedron or p10 promotersin vivo may not occur until an even later time as there is some doubt asto the level of expression of genes under the control of the viralpolyhedron or p10 promoters in the epithelial cells of the midgut, theprimary site of infection, as normal production of polyhedra is notobserved in these cells (Granados and Lawler, 1981). Thus, placing aninsect incapacitating or toxic gene under the control of the polyhedronor p10 promoter may offer modest advantages in the order of only 1 or 2days in terms of accelerating insect death relative to an infection witha wild type baculovirus.

Because early viral promoters are usually essential and cannot bedeleted, their utilization as the promoter for the toxin gene wouldrequire the presence of a duplication of the promoter sequence in theviral genome. Recombinant viruses containing such duplications of earlyviral promoters may prove unstable over the many large-scale passagesnecessary for commercial production.

A recombinant virus has also been developed which contains silkmothchorion chromosomal genes under the control of their own promoter(Iatrou and Meidinger, 1990). The transcripts from this recombinantvirus are expressed correctly only in the tissue in which this promoteris normally active, ovarian follicular cells of the insect, but not inany other tissues, for example fat body, other tissues of the abdomensuch as muscles or ganglia or in non-expressing tissue culture cellssuch as Bm5 cells. Further, because of the presence of a thick basementmembrane that completely surrounds each follicle, the later recombinantvirus infects the follicle cells only in a limited fashion and onlyafter a considerable time lag (e.g., 36-48 hours) after in vitroinoculation (injection) of the insect with the virus.

Advances in the genetics of invertebrate viruses and cells have allowedthe development of viral-cellular systems which give both a high levelof synthesis and complex processing of recombinant products. Inparticular baculoviruses such a Autographica californicanucleopolyhedrosis virus (AcNPV) and Bombyx mori (BmNPV)nucleopolyhedrosis virus are extremely useful helper-independenteukaryotic vectors. Both these systems are based on the utilization ofthe strong promoter of the gene encoding polyhedron. The techniquesconventionally employed in these systems are described in U.S. Pat. No.4,745,051 and U.S. Pat. No. 5,194,376 both of which are incorporated byreference in their entirety herein. This system has been used for thesuccessful production of large quantities of many different geneproducts. One difficulty with this system is the cells eventually diebecause they are infected with a virus.

The hr's are repeated sequences present in several baculoviruses,including Autographica californica nuclear polyhedrosis virus (AcNPV)(Cochran and Faulkner, 1983; Guarino and Summers, 1986; Guarino et al.,1986) and BmNPV (Maeda and Majima, 1990; Kamita et al., 1993). The hrelements have been shown to serve as origins of replication in AcNPV andcan, under some conditions, allow plasmids containing these sequences toreplicate in AcNPV-infected cells (Pearson et al., 1992; Kool et al.,1993). The hr's of AcNPV (designated hr1 through hr5) have beenpreviously shown to serve as strong enhancers for early viral genes suchas the 39K gene (Guarino and Summers, 1986; Guarino et al, 1986), the35K gene (Guarino and Summers, 1987; Nissen and Friesen, 1989), and theIE-N gene (Carson et al., 1991). Expression of other baculovirus genes,however, such as the IE-1 gene (Guarino and Summers, 1986) and thepolyhedron gene, is not stimulated by the hr elements. The AcNPV hr5 hasalso been demonstrated to enhance a promoter of non-baculovirus origin,the Rous Sarcoma Virus long terminal repeat (RSV LTR) promoter (Guarinoand Summers, 1986). In the case of the RSV LTR promoter, the 35promoter, (Nissen and Friesen, 1989), and the IE-N promoter, hrenhancers were able to stimulate transcription in the absence of theIE-1 gene product. The hr enhancers were only able to stimulatetranscription from the 39K promoter, however, in the presence of theIE-1 protein. An hr enhancer-binding protein was detected in insectcells after transfection with the AcNPV IE-1 gene, but no bindingactivity could be detected in normal cells (Guarino and Dong, 1991).

The IE-1 gene product is a gene product which is expressed by thebaculovirus genome at the early stages of infection under the control ofthe transcriptional machinery of the insect cell. Upon expression thegene product stimulates the expression of the p39 and IE-N genes of thebaculovirus genome (Carson et al., 1988).

SUMMARY OF THE INVENTION

In one of its method aspects the invention is directed to a method ofincapacitating insects comprising infecting an insect with an activerecombinant baculovirus, such baculovirus comprising a structural geneencoding an incompatible protein functionally linked to an insectcellular promoter and an enhancer under conditions where theincompatible protein is expressed in the insect having the recombinantbaculovirus present therein.

In the second of its method aspects the invention is directed to amethod of producing heterologous protein in insect cells comprisingexpressing the heterologous protein from a enhanced recombinantexpression cassette such cassette comprising a structural gene encodingthe heterologous protein functionally linked to an insect cellularpromoter and an enhancer.

In a third method aspect, the invention is directed to a method ofproducing heterologous protein in insect cells wherein the insect cellscomprise an IE-1 gene under conditions wherein the IE-1 product isproduced, such method comprising expressing the heterologous proteinfrom a recombinant expression cassette, such cassette comprising astructural gene encoding a protein functionally linked to an insectcellular promoter.

In one of its product aspects the invention is directed to an expressioncassette comprising an insect cellular promoter and an enhancer whereinthe insect cellular promoter is capable of expressing a heterologousgene functionally linked to the promoter. The invention is also directedto recombinant expression cassettes, transplacement fragments andtransplacement vectors having the enhanced expression cassette.

In another of its product aspects, the invention is directed to aninsect cell comprising the IE-1 gene in the absence of addedbaculovirus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—Plasmid Vectors Used in the Generation of Recombinant Viruses

A. This is a schematic representation of the vector pBmA. The 5′ and 3′flanking sequences of the cytoplasmic actin gene are represented by theunfilled regions and the transcribed sequences are indicated by theshaded region. The line represents sequences from pBS/SK+ containing thebeta-lactamase (bla) gene which confers ampicillin resistance. All ofthe restriction enzyme sites shown in the multiple cloning site areunique in this plasmid.

B. This is a schematic representation of the vector pBmA.cat. The regioncontaining vertical lines represents the cat gene open-reading frame.All other designations are as in A.

C. This is a schematic representation of the vector pBmp2s. The regioncontaining horizontal lines represents the deleted polyhedron genecontaining no promoter sequences. Black regions represent sequencesflanking the polyhedron gene in the BmNPV genome. The line representspUC9 DNA.

D. This is a schematic representation of the vector pBmp2s/A.cat. Theline represents DNA from pBS/SK+. All other designations are asdescribed in A and C.

E. This is a schematic representation of pBmp26.cat. The regioncontaining horizontal lines represents the polyhedron gene including allsequences necessary for strong promoter activity. All other designationsare as described in C.

FIG. 2—Analysis of CAT Activity in Non-Occluded Virus

A. Non-occluded virus from cells transfected with pBmNPV/A.cat orpBmNPV/P26.cat was separated from the medium as described in Example 2and the level of CAT determined. The upper spots (Ac-C) represent acetylchloramphenicol, the products of the CAT reaction, while the lower spotin each assay (C) represents the substrate, chloramphenicol.

B. Bm5 cells were assayed for CAT activity at the indicated times afteraddition of BmNPV/A.cat or BmNPV/P26.cat inoculum. Assays were performedusing the amount of cell protein and length of incubation indicated andas described in Example 5.

C. Bm5 cells were infected with the standard inoculum containing mediumand virus (M+V), with non-occluded virus isolated from the inoculum (V),or mock infected (- - -). The cells were collected 1 hour afterinfection and were washed once (1×) or five times (5×) with 1 ml PBS.

FIG. 3—Time Course of CAT Activity in Infected Bm5 Cells

Cells were infected and assayed as described in Example 6.

A. Cells were infected with BmNPV/A.cat inoculum and CAT assays wereperformed at the indicated time post infection using 5 μg cell proteinincubated for 1 hour.

B. Cells were infected with BmNPV/P26.cat inoculum and assayed using 1μg cell protein incubated for 1 hour at the indicated times postinfection.

C. Appropriate amounts of cell extract from cells infected withBmNPV/A.cat (actin) or BmNPV/P26.cat (polyhedron) were assayed toquantitate CAT assays at each time point. The background CAT activityobserved at 1-2 hours p.i. was subtracted from each point. The valueswere plotted on a logarithmic scale.

FIG. 4—Analysis of CAT Activity in Infected Larvae

Fifth instar larvae were injected with either BmNPV/A.cat orBmNPV/P26.cat inoculum and after 2 days were assayed for CAT activity inbody wall (B), head (H), midgut (M), silk gland (S), and gonad (G) asdescribed in Example 7. Tissues from three larvae were pooled for eachassay.

FIG. 5—Time Course of CAT Activity in Infected Larvae

Early fifth larvae were injected with either BmNPV/A.cat orBmNPV/P26.cat inoculum and were assayed for CAT activity in body walltissues as described in Example 7 at various times p.i. Each pointrepresents the average of two larvae. (Ac-C represents acetylchloramphenicol; C is the substrate chloramphenicol.)

A. CAT assays of body wall tissues from larvae injected withBmNPV/A.cat.

B. CAT assays of body wall tissues from larvae injected withBmNPV/P26.cat.

C. Appropriate amounts of body wall tissue from larvae injected withBmNPV/A.cat (solid circles) or BmNPV/P.26cat (open circles) were assayedfor CAT activity, the background level of CAT activity observed at 2-4hr p.i. was subtracted and the values plotted on logarithmic scale.

FIG. 6—Transfection of Cells with pBmA.cat

Tissue culture cells (Bm5 or Sf21) were transfected as described inExample 8. (Ac-C represents acetyl chloramphenicol; C represents thesubstrate chloramphenicol).

A. CAT assays performed on the indicated amount of culture cells 2 daysafter cells were transfected with pBmA.cat.

B. The amount of plasmid DNA present in the transfected cells wasmeasured by dot-blot hybridization of the indicated amount of cellsuspensions, using as a probe radioactively labelled pBS/SK+ DNA, asdescribed in Example 8.

FIG. 7—Southern Blot Analysis of the Contents of Polyhedra from MixedInfections

This is an autoradiogram of DNA from Bm5 cells infected with virusreleased from occlusion bodies from cells infected with mixtures of purewild type BmNPV (“WT”) and BmNPV/A.cat (“A.cat”) or pure wild type BmNPV(“WT”)BmNPV/P26.cat (“P26.cat”). The lines designated A.cat and P26.catalone are controls.

FIG. 8—Time Course of CAT activity in orally infected larvae

A. Gut (“G”) from larvae infected with BmNPV/A.cat (“A.cat”) orBmNPV/P26.cat (“P26.cat”) were analysed at the indicated times for CATactivity. (Ac-C represents acetyl chloramphenicol; C represents thesubstrate chloramphenicol)

B. Mock infected (“MI”) and body wall tissues (“BW”) from larvaeinfected with BmNPV/A.cat (“A.cat”) or BmNPV/P26.cat (“P26.cat”) wereanalysed at the indicated times for CAT activity. (Ac-C representsacetyl chloramphenicol; C represents the substrate chloramphenicol)

FIG. 9—Location of hr3 in the BmNPV genome and its nucleotide sequence.

A. Physical map of part of the BmNPV genome showing restriction sites.Arrows indicate gene products GC30 and Vp39 from baculovirus genome.

B. The nucleotide sequence of the 1.2 kb Sspl fragment containing thehr3 sequence (SEQ ID NO:1). Box indicates region with homology to hrsequences. DNA sequence for EcoRI sites are italicized.

FIG. 10—Enhancement of the actin promoter in transfected cells.

A. Plasmids used for transfection containing the actin promoter (P_(A)),the chloramphenicol acetyl transferase coding region (cat), the actinterminator (T_(A)), and the 1.2 kb enhancer containing sequence (E). Thearrow indicates the orientation of the enhancer sequence, as indicatedin FIG. 9A.

B. CAT activity in Bm5 cells transfected with the plasmids in FIG. 10Aand pBmA.cat. (Ac-C represents acetyl chloramphenicol; C represents thesubstrate chloramphenicol)

C. CAT activity in Sf21 cells transfected with the plasmids pBmA.cat andp13315. (Ac-C represents acetyl chloramphenicol; C represents thesubstrate chloramphenicol)

FIG. 11—Enhancement of the actin promoter in infected cells.

A. CAT activity of cells infected with BmNPV and transfected withpBmA.cat (“A.cat”) or p13315 (“eA.cat”). (Ac-C represents acetylchloramphenicol; C represents the substrate chloramphenicol)

B. Autoradiogram of DNA from nuclei of insect cells infected with BmNPVand transfected with pBmA.cat (“A.cat”) or p13315 (“eA.cat”). Numbersindicate the number of nuclei dot blotted onto the membrane.

FIG. 12—Plasmids used in the generation of recombinant virus.

A. This is map of the plasmid pBMA. Vertical lines indicate the codingregion of the B. mori cytoplasmic actin gene (A3) and the arrowindicates the direction of transcription. The single line indicatespBS/SK+ sequences including the β-lactamase (bla) gene.

B. This is a map of the plasmid pBmeA. The 1.2 kb enhancer fragment islabelled “E” (shaded region). The other labels are as indicated in FIG.12A.

FIG. 13—CAT assays of insect cells transfected with A.cat and eitherAcIE-1 or BmIE-1

A. CAT analysis of Bm5 cells transfected with pBmpA.cat alone, pBmpA.catand pBmIE-1 or pBmpA.cat and pAcIE-1. (Ac-C represents acetylchloramphenicol; C represents the substrate chloramphenicol)

B. CAT analysis of Sf21 cells transfected with pBmpA.cat alone,pBmpA.cat and pBmIE-1 or pBmpA.cat and pAcIE-1. (Ac-C represents acetylchloramphenicol; C represents the substrate chloramphenicol)

FIG. 14—CAT assays of insect cells transfected with different plasmids

“A.CAT” indicates the presence of the actin promoter, “pBmIE1” indicatesthe presence of the IE-1 structural gene and “enhancer” indicates thepresence of the 1.2 kb enhancer element in the cells. The boxes indicatewhether the elements are present on the same plasmid (inside the samebox) or on different plasmids (i.e. different boxes). (Ac-C representsacetyl chloramphenicol; C represents the substrate chloramphenicol)

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the present invention generally relates to a method ofexpressing heterologous proteins in insect cells using genetic elementswhich potentiate activity of an insect cellular promoter basedexpression cassette functionally attached to a structural gene for aheterologous protein. The heterologous protein may be a heterologousprotein which is toxic to the insect or any other heterologous proteinwhose unregulated expression could incapacitate the insect host throughunbalancing of an important physiological process. This invention isalso related to expression cassettes containing insect cellularpromoters and enhancers, recombinant expression cassettes containingheterologous genes functionally attached to insect cellular promoters,transplacement fragments containing recombinant expression cassettes,vectors having transplacement fragments, recombinant baculoviruses andinsect cells derived therefrom.

The first genetic element is an enhancer which can stimulate thecellular promoter upon covalent linkage to the promoter. It has beenfound that depending on the specific promoter-enhancer linkageconfiguration the level of promoter activity potentiation relative tothe activity of a recombinant expression cassette that lacks theenhancer is from about 40 fold to 100 fold. Further it has been foundthat transcriptional potentiation in insect cells is independent ofviral infection. Rather the enhancers effect is mediated by cellularrather than viral factors. Further, the enhancer is equally active incell lines derived from different lepidopteran insect species.

The second genetic element is a structural gene, IE-1, of thebaculovirus genome which is expressed at the early stages of infectionunder the control of the transcriptional machinery of the cell. It hasbeen found that this gene encodes a protein which acts as atranscriptional regulator. Upon expression, this protein stimulates thelevel of expression of heterologous gene products directed by thecellular promoter based expression cassette. This stimulation occurs intrans. i.e. the enhancing effect is the same irrespective of whether thegene encoding IE-1 is physically linked to the cellular promoter of theexpression cassette in a vector or not. The enhancing effect of the IE-1product on the cellular promoter in non-infected insect cells isapproximately 100 fold. It has further been found that this effect isindependent of the presence of the enhancer. Furthermore, the IE-1 geneis equally active in cell lines derived from a number of lepidopteraninsects, for example Bombyx mori and Autographica californica. Also theIE-1 gene of AcNPV can substitute for that of BmNPV.

Addition of both genetic elements to the cellular promoter basedexpression cassette (the enhancer linked to the expression cassette andthe IE-1 gene linked to the expression cassette or supplied separatelyto the cells in the form of a second plasmid) results in an increase inthe expression of the heterologous proteins encoded by genesfunctionally linked to the cellular promoter.

This discovery has dual applications. The first utility is in the areaof continuous high level expression of foreign genes in insect linestransformed with enhanced recombinant expression cassettes employingcellular promoters functionally linked to the foreign gene. To achievecontinuous high level of expression of foreign genes, normal insecttissue culture cells can be transformed with a plasmid containing anenhanced expression cassette comprising a cellular promoter and theenhancer functionally linked to a foreign gene and an extra geneexpressing a selective marker (e.g. antibiotic resistance gene under thecontrol of a promoter that functions constitutively in insect cells, forexample, the promoter of the IE-1 gene). Application of a relevantselection should lead to integration of one or more multiple copies ofthe plasmid into the chromosomes of the cells, thus generating an insectcell line capable of continuous high level expression of the foreigngene present in the recombinant expression cassette.

Alternatively, the insect cells can be transformed with a plasmidcontaining the IE-1 gene and a suitable resistance gene, thus generatinga cell line expressing continuously the IE-1 gene product. Such a cellline can be subsequently transformed with additional plasmids containingthe either the basic recombinant expression cassette or the enhancedrecombinant expression cassettes and an additional gene conferringresistance to a second selection agent. In both cases, synthesis of theforeign protein will be continuous, because integrated expressioncassettes cannot be lost through replication and the insect cells neverdie because they are not infected by any viruses. Expression will alsooccur at a high level.

The second application of this invention is in the development ofrecombinant baculoviruses for use as insect pest control agents. Thediscovery and utilization of the enhancer represents a significantimprovement on the use of recombinant baculoviruses containing cellularpromoter based expression cassettes. The use of the enhancer allows forenhanced, immediate and ubiquitous expression of foreign products fromgenes cloned into enhanced expression cassettes in insect infected withthe corresponding enhanced recombinant baculoviruses. Only linkage ofthe enhancer to the cellular promoter is required in the case ofrecombinant baculoviruses since the IE-1 gene is produced in theinfected cells by the baculovirus' genome immediately upon infection.

However, prior to discussing this invention in detail, the followingterms will first be defined.

Definitions

The term “baculovirus” is used herein as an alternative to the term“nuclear polyhedrosis virus” or “NPV”. It encompasses viruses classifiedunder subgroup A of the family of Baculoviridae. Preferably it includesthe viruses specific for the following insects: Bombyx sp. Autographicasp. and Spodoptera sp.

The term “expression cassette” means a fragment of nucleic acidcomprising an insect cellular promoter sequence, with or without asequence containing mRNA polyadenylation signals, and one or morerestriction enzyme sites located downstream from the promoter allowinginsertion of heterologous gene sequences. The expression cassette iscapable of directing the expression of a heterologous protein when thestructural gene encoding the heterologous protein is functionallyattached to the insect cellular promoter by insertion into one of therestriction sites. The recombinant expression cassette allows expressionof the heterologous protein in an insect when the expression cassettecontaining the heterologous protein is introduced into the tissues ofthe insect. Preferably the recombinant expression cassette allowsexpression at an early stage of infection and/or it allows expression insubstantially all tissues of an insect. In one embodiment the expressioncassette is that present in the plasmid pBmA.

The term “enhanced expression cassette” means a fragment of nucleic acidcomprising an insect cellular promoter sequence and an enhancer, with orwithout a sequence containing mRNA polyadenylation signals, and one ormore restriction enzyme sites located downstream from the promoterallowing insertion of heterologous gene sequences. The enhancedexpression cassette is capable of directing the expression of aheterologous protein when the structural gene encoding the heterologousprotein is functionally attached to the insect cellular promoter and theenhancer by insertion into one of the restriction sites. Preferably, theenhanced expression cassette is that DNA fragment present on plasmidpBmeA.

The term “recombinant expression cassette” means an expression cassettefurther comprising a structural gene sequence encoding a heterologousprotein inserted into a restriction enzyme site such that the structuralgene sequence is functionally linked to the insect cellular promoter.When the recombinant expression cassette is inserted into insect cells,the heterologous protein is expressed under the control of the insectcellular promoter. Preferably the recombinant expression cassette allowsexpression at an early stage of infection and/or it allows expression insubstantially all tissues of an insect.

The term “recombinant enhanced expression cassette” means an enhancedexpression cassette further comprising a structural gene sequenceencoding a heterologous protein inserted into a restriction enzyme sitesuch that the structural gene sequence is functionally linked to theinsect cellular promoter and the enhancer. When the recombinantexpression cassette is inserted into insect cells, the heterologousprotein is expressed under the control of the insect cellular promoterand the enhancer.

The term “transplacement fragment” means a DNA fragment which comprises:(1) an expression cassette sequence or a recombinant expression cassettesequence, and (2) a portion of a baculovirus genome that can sustaininsertions of non-viral DNA fragments. The term “enhanced transplacementfragment” means a DNA fragment which comprises: (1) an enhancedexpression cassette sequence or a recombinant enhanced expressioncassette sequence, and (2) a portion of a baculovirus genome that cansustain insertions of non-viral DNA fragments. The term “a portion of abaculovirus genome that can sustain insertions of non-viral DNAfragments” means a portion of the genome into which non-viral DNAfragments can be inserted or which can be replaced with the non-viralDNA fragments without affecting viral infectivity, replication orassembly. This portion of the genome should be of sufficient size toallow recombination events to occur between the transplacement vector orenhanced transplacement vector and a wild type baculovirus genome suchthat the expression cassette or enhanced expression cassette is insertedinto the genome. One skilled in the art would know the size of thebaculovirus flanking sequences necessary to allow recombination events.Preferably, the size of the baculovirus flanking sequences are at leastabout 500 bp on each side of the expression cassette, more preferably,the size of the flanking sequences is at least about 5,000 bp on eachside.

In one embodiment, the sequence from the baculovirus genome comprisesthe 5′ and 3′ sequences of the polyhedron gene of the B. moribaculovirus. In another embodiment, the portion of the baculovirusgenome contains the polyhedron gene and flanking sequences of thebaculovirus AcNPV.

The term “transplacement vector” means nucleic acid which comprises: (1)a transplacement fragment, and (2) DNA sequences allowing replicationand selection in bacteria, for example E. coli. An “enhancedtransplacement vector” comprises an enhanced transplacement fragment andthe bacterial DNA sequences. The vector may be a plasmid, another virusor simply a linear DNA fragment. A transplacement fragment or vector isused to produce recombinant baculoviruses through doublerecombination/cross-over events. When insect cells are transfected withthe transplacement vector and DNA from wild-type baculovirus, a doublecross-over event between the homologous portions of the baculovirusgenome and the transplacement fragment will result in the replacement ofa portion of the wild-type baculovirus sequence with the part of thetransplacement fragment which contains the recombinant expressioncassette.

The term “recombinant baculovirus” refers to a baculovirus whose genomecomprises a recombinant expression cassette of the invention. An“enhanced recombinant baculovirus” means a baculovirus comprising arecombinant enhanced expression cassette. In one embodiment, therecombinant baculovirus comprises B. mori nuclear polyhedrosis virus inwhich a section of the DNA sequence encoding the polyhedron gene isreplaced with a transplacement fragment comprising a heterologousstructural gene functionally linked to an insect cellular promoter,preferably the promoter for the cytoplasmic actin gene of B. mori. Inanother embodiment, the recombinant baculovirus comprises A. californicanuclear polyhedrosis virus in which a transplacement fragment isinserted into a position on the genome 40 bp upstream of the polyhedrongene.

The term “promoter” means a DNA sequence which initiates and directs thetranscription of a heterologous gene into an RNA transcript in cells.

An “insect cellular promoter” is a promoter which will direct theexpression of a heterologous structural gene when the gene isfunctionally linked to the insect cellular promoter in a recombinantexpression cassette and the recombinant expression cassette isintroduced into insect cells either by transfection of the cellsdirectly or with a transplacement vector comprising the expressioncassette and wild-type baculovirus DNA or by infection of the cells witha recombinant baculovirus comprising the expression cassette. Preferablythe insect cellular promoter allows expression at an early stage ofinfection and/or it allows expression in substantially all tissues of aninsect. In a preferred embodiment, the “insect cellular promoter” is apromoter which does not normally direct the expression of theheterologous structural gene and is not naturally functionally attachedto that structural gene. For example, if the heterologous structuralgene is a gene naturally present in the insect genome, the insectcellular promoter is not the promoter which normally directs theexpression of the heterologous structural gene in the wild-type insect.In one embodiment, the insect cellular promoter is an insect cytoplasmicactin promoter, most preferably, the insect cellular promoter is thecytoplasmic actin promoter of B. mori. In another embodiment, the insectcellular promoter comprises the promoter for an insect ribosomal gene,tRNA gene, histone gene, or tubulin gene.

The term “enhancer” means a nucleic acid sequence that increases thefrequency with which transcription initiates from a promoterfunctionally linked to the enhancer. The enhancer can function in anylocation, either upstream or downstream, relative to the promoter. Theenhancer in this invention is any DNA sequence which is capable ofincreasing the level of transcription from the insect cellular promoterwhen the enhancer is functionally linked to the promoter. One skilled inthe art, given the present disclosure, could readily determine whether aparticular DNA fragment functioned as an enhancer by inserting thefragment next to the cellular promoter and measuring the level ofproduction of mRNA from the promoter. In a preferred embodiment theenhancer is the 1.2 kb DNA fragment shown in FIG. 9. More preferably,the enhancer is that region of the 1.2 kb BmNPV enhancer fragment whichpotentiates the transcription from the cellular promoter. One skilled inthe art given the disclosure of this invention could readily determinethe size of fragment from the 1.2 kb DNA fragment shown in FIG. 9 whichpotentiates the transcription from the cellular promoter by insertingfragments of differing sizes from that DNA fragment into an expressioncassette of the present invention and determining by the methods of thisinvention whether increased production of a heterologous protein wasachieved.

Preferably, the level of transcription is enhanced from the cellularpromoter by the enhancer when transcription is increased by a level ofat least about 10 fold, more preferably the level of transcription isincreased by a level of at least about 100 fold.

In order to enhance the efficiency of the expression of the heterologousgene, it is contemplated that the insect cellular promoter may begenetically modified so as to make it capable of expressing theheterologous gene more efficiently in the cells of the insect brain, gutand/or muscle. In addition, it is contemplated that the enhancer mayalso be genetically modified to further enhance expression. In addition,the use of a tissue-specific “enhancer” element or some other DNAsequence is contemplated such that, while the promoter is expressed insubstantially all tissues, it is over-expressed in certain tissues ofthe insect.

It is further contemplated that the expression cassette will include aDNA fragment encoding a signal peptide sequence functionally linked tothe heterologous gene for the purposes of directing secretion of theheterologous protein out of the insect cell. In this case, the signalsequence must be linked in frame with the open reading frame of theheterologous gene.

The term “functionally linked” or “functionally attached” whendescribing the relationship between two DNA regions simply means thatthey are functionally related to each other and they are located on thesame nucleic acid fragment. A promoter is functionally attached to astructural gene if it controls the transcription of the gene and it islocated on the same nucleic acid fragment as the gene. An enhancer isfunctionally linked to a structural gene if it enhances thetranscription of that gene and it is functionally located on the samenucleic acid fragment as the gene.

The term “IE-1 gene” refers to the IE-1 gene from a baculovirus genome.(Huybrechts et al., 1992) In one embodiment, the IE-1 gene is obtainedfrom the BmNPV genome. In another embodiment, the IE-1 gene is obtainedfrom the AcNPV genome.

The term “constitutive expression” means that the promoter is expressedcontinuously in the insect cells. In the case of insect cells into whichrecombinant virus has been introduced, the promoter is expressed for atleast 20 hours after introduction, more preferably for at least 30 hoursafter introduction and most preferably at least 60 hours afterintroduction.

The term “at an early stage of introduction” means production of theheterologous protein under the functional control of the insect cellularpromoter occurs before expression would occur if the heterologousprotein was functionally attached to a polyhedron promoter or to otherviral promoters that are functional after viral DNA replication. Whereinsect tissue culture cells are infected with recombinant virus,expression occurs at an early stage of introduction where theheterologous protein is produced before about 10 hours, more preferablybefore about 8 hours, most preferably before about 5 hours postinfection. Where insect larvae are infected with recombinant viruscontaining the expression cassette, expression will occur at an earlystage of introduction when it occurs before about 48 hours postinfection, more preferably before 24 hours post infection and mostpreferably before 12 hrs post infection.

The term “in substantially all insect tissues” means that theheterologous protein is expressed in all insect tissues, more preferablythe gut, brain, nervous system, fat body and muscle tissue, into whichit is introduced by infection with a recombinant baculovirus comprisinga structural gene coding for the heterologous protein functionallylinked to an insect cellular promoter.

The terms “producing heterologous protein” or “expressing heterologousprotein” means that the structural gene encoding the heterologousprotein is transcribed into mRNA and that the mRNA is further translatedinto protein. In a preferred embodiment the heterologous protein will beproperly processed by the insect cell, although such processing may bein a tissue specific manner.

The term “structural gene” refers to those DNA sequences which, whenfunctionally attached to a cellular promoter, will be transcribed andproduce a heterologous protein in insect cells.

The term “heterologous structural gene” or “heterologous gene” is astructural gene which is not normally present in wild-type baculovirusgenomes, but which may or may not be present in insect genomes. In apreferred embodiment the heterologous structural gene does not includethe structural gene which is functionally attached to the insectcellular promoter in wild-type insect cells. A heterologous structuralgene is a structural gene which will be transcribed and will produce aprotein when functionally attached to an insect cellular promoter in arecombinant expression cassette or to an enhancer and promoter in arecombinant enhanced expression cassette and thereafter introduced intocells of an insect either by infection of cells by a recombinantbaculovirus containing the cassette or by transfection of cells with atransplacement fragment containing the cassette or with the recombinantexpression cassette alone. While the chloramphenicol acetyltransferase(“CAT”) gene was used to characterize the expression of the heterologousprotein under the control of the cellular promoter in the examplesprovided herein, it will be recognized that any heterologous structuralgene meeting the above criteria may be used in the invention.

The term “heterologous protein” refers to a protein encoded by aheterologous structural gene and which is not normally expressed by thebaculovirus, but which may be expressed by insect cells in a regulatedmanner. The protein may be compatible or incompatible with the insect.Examples of compatible heterologous proteins are chloramphenicolacetyltransferase, human alpha interferon (IFN-α), insulin-like growthfactor-II (IGF-II), human interleukin 3, mouse interleukin 3, human andmouse interleukin 4, human T-lymphotropic virus (HTLV-1) p40^(x), HTLV-1env, human immunodeficiency virus (HIV-1) gag, pol, sor, gp41, andgp120, adenovirus E1a, Japanese encephalitis virus env (N), bovinepapillomavirus 1 (BPV1) E2, HPV6b E2, BPV1 E6, and human apolipoproteinsA and E; β-galactosidase, hepatitis B surface antigen, HIV-1 env, HIV-1gag, HTLV-1 p40^(x), human IFN-β, human interleukin 2, c-myc, D.melanogaster Kruppel gene product, bluetongue virus VP2 and VP3, humanparainfluenza virus hemagglutinin (HA), influenza polymerases PA, PB1,and PB2, influenza virus HA, lymphocytic choriomeningitis virus (LCMV)GPC and N proteins, Neurospora crassa activator protein, polyomavirus Tantigen, simian virus 40 (SV40) small t antigen, SV40 large T antigen,Punta Toro phlebovirus N and Ns proteins, simian rotavirus VP6, CD4(T4), human erythropoietin, Hantaan virus structural protein, humanepidermal growth factor (EGF) receptor, human insulin receptor, human Blymphotrophic virus 130-kd protein, hepatitis A virus VP1, humantyrosine hydroxylase, human glucocerebrosidase, and mouse p53.

The term “incompatible protein” means either a toxic protein or aninsect incapacitating protein of insect or non-insect origin whoseunregulated expression could incapacitate the insect through unbalancingof an important physiological process. An incompatible protein willenhance the inactivation of the insect by the baculovirus.

The term “toxic gene” means a heterologous DNA sequence which encodesfor a heterologous protein product that inactivates the larvae or whichresults in extensive tissue damage to the larvae and eventually death.Suitable toxic genes include genes that encode insect specific toxins orother gene products which, when inserted into the expression cassetteimprove the ability of the baculovirus to paralyze or kill the insect.Such toxic genes include genes which encode for endotoxins from Bacillusthuringiensis. There are B. thuringiensis strains with activitiesagainst a wide range of insect species. Many strains produce a toxinthat is active against lepidopteran larvae. Some strains produce toxinsthat affect dipteran or coleopteran larvae. Other suitable toxic genesinclude genes encoding: an insect specific neurotoxin, AaIT, from thevenom of the North African (Algerian) Scorpion, Androctonus australisHector (Stewart et al., 1991); Tox-34, which encodes TxP-1 toxinassociated with the mite Pyemotes tritici. (Tomalski and Miller, 1991);the depressant insect toxin BjIT2 of the scorpion Buthotus judaicus(Zilberg, N., et al., 1991, Toxicon, 29:1155-1158); an alpha insecttoxin from the scorpion Leirus quinquestriatus hebraeus (Gurevitz, M.,et al., 1991, Toxicon, 29:1270-1272); and insect paralytic peptides(Skinner W., et al., 1991, J. Biol. Chem., 20:12873-12877).

The term “incapacitating protein” means a protein whose unregulatedexpression could incapacitate the insect through unbalancing of animportant physiological process. Examples of incapacitating genesinclude: insect or non-insect neuropeptides such as FMRFamide-relatedpeptides, enkephalin-related peptides, tachykinins, adipokinetichormones, other myotropic peptides, such as proctolin, leucokinins,achetakinins, drosulfakinins, locustakinin, locustamyotropins,leucopyrokinin, locustapyrokinin, leucosulfakinin, locustasulfakinin,allatostatins, allatotropin, alkine phosphatases, collagenases,chitinases, juvenile hormone esterases and epoxide hydrolases, ecdysoneand juvenile hormone receptors, eclosion hormone, prothoracicotropichormone and the synthetic diuretic hormone gene of M. sexta.

The term “introduction” refers to either infection or transfection ofinsect cells.

Insects are “incapacitated” when substantially all of the insects arekilled. In a preferred embodiment at least about 50% of the insects arekilled, more preferably at least about 75% of the insects are killed.

The term “infection” refers to the invasion by pathogenic viral agentsof cells where conditions are favorable for their replication andgrowth. Such invasion can be by placing the vital particles directly onthe insect cell culture or by injection of the insect larvae with therecombinant virus or by oral ingestion of the viral particles by theinsect. The amount of recombinant virus injected into the larvae will befrom 10² to 10⁵ pfu of non-occluded virus/larvae. Alternatively, larvaecan be infected by the oral route using occlusion bodies carryingrecombinant viruses. In general, the amount of occlusion bodies fed tothe larvae is that amount which corresponds to the LD₅₀ for that speciesof baculovirus and insect host. The LD₅₀ varies with each species ofbaculovirus and the age of the larvae. One skilled in the art canreadily determine the amount of occlusion bodies to be administered.Typically, the amount will vary from 10-10⁶ occlusion bodies/insect.

In those vectors in which the polyhedron gene has been replaced by theexpression cassette, the recombinant virus does not produce thepolyhedron protein, and occlusion body formation does not occur.However, co-infection of cell cultures with the budded virus forms ofboth recombinant and wild-type viruses provides occlusion bodiescontaining both recombinant and wild-type viruses. This mixture of bothrecombinant and wild-type viruses in occlusion bodies bay be used forinfection of insect larvae by the oral route. Alternatively, byconstructing different transplacement vectors, it is possible to insertthe recombinant expression cassette in other sites of the viral genometo generate occluded recombinant baculoviruses.

The term “transfection” refers to a technique for introducing purifiednucleic acids into cells. Calcium phosphate or other appropriate agentssuch as dextran sulfate are added to the DNA solution and the solutionis placed on the insect cells. The insect cells will take up the DNAprecipitated by the addition of calcium phosphate or other agents suchas dextran sulphate to a DNA solution. Alternatively, DNA can beintroduced into the cells by electroporation. In a preferred embodiment,the DNA is introduced into the cells by mixing the DNA solution withLipofectin™ (GIBCO BRL Canada, Burlington, Ontario) and adding themixture to the cells.

Once an appropriate transplacement fragment or transplacement vectorcontaining the heterologous gene is constructed, host insect cells aretransfected simultaneously with wild-type viral DNA and thetransplacement fragment or vector DNA containing baculovirus DNAsequences homologous to the wild-type viral DNA. The geneticrecombination system of the host insect cell recombines the plasmid andviral DNAs. Double crossover recombination events at homologous DNAsites results in the replacement of sequences of the viral genome with aportion of the transplacement fragment DNA, thereby inserting therecombinant expression cassette DNA containing the heterologous geneinto the preferred site of the viral genome. Where the transplacementfragment or vector contains DNA sequences from the viral polyhedrongene, a double recombination/cross-over event between the homologousviral sequences in the transplacement fragment and the wild-typebaculovirus genome will result in the expression cassette replacing aportion of the polyhedron gene of the wild-type genome. Followingamplification of serially diluted progeny viruses, recombinant virusesare selected by hybridization to heterologous gene probes and confirmedby restriction endonuclease and DNA sequence analysis identificationtechniques. In the case of polyhedron substitution expression systems,cells containing recombinant viruses with double crossovers can be alsoidentified visually because they do not contain viral occlusion bodies.

The term “insect cells” means insect cells from the insect species whichare subject to baculovirus infection. For example: Autographacalifornica; Bombyx mori; Spodoptera frugiperda; Choristoneurafumiferana; Heliothis virescens; Heliothis zea; Orgyia pseudotsugata;Lymantria dispar, Plutella xylostella; Malacostoma disstria;Trichoplusia ni; Pieris rapae; Mamestra configurata; Hyalophoracecropia.

Methodology

In view of the fact that ferocious feeding is the major destructiveactivity of most lepidopteran pests, recombinant baculoviruses thateffect an immediate incapacitation of pests by early expression of theincapacitating genes in the cells of the midgut would be advantageous.It has now been discovered that there are genetic elements whichpotentiate activity of an insect cellular promoter functionally attachedto a structural gene for a heterologous protein. Further, the protein isexpressed early in the baculovirus infection cycle and in substantiallyall infected tissues of the insect including the gut. Therefore, if aheterologous gene whose product incapacitates the insect is placed underthe control of an insect cellular promoter in a recombinant baculovirus,the gene product will be expressed in substantially all infected tissuesof the insect at an early stage of infection and thus rapidly inactivatethe insect.

The first genetic element is an enhancer which can stimulate thecellular promoter upon covalent linkage to the promoter. The secondgenetic element is a structural gene, IE-1, of the baculovirus genomewhich is expressed at the early stages of infection under the control ofthe transcriptional machinery of the cell. It has been found that thisgene encodes a protein which acts as a transcriptional regulator. Uponexpression, this protein stimulates the level of expression of thecellular promoter of the recombinant expression cassette. Thisstimulation occurs in trans. i.e. the enhancing effect is the sameirrespective of whether the gene encoding IE-1 is physically linked tothe cellular promoter of the expression cassette in a vector or not.

The present invention is directed, in part, to the discovery of anenhanced expression cassette which can be used to generate recombinantbaculoviruses that can express heterologous proteins under the controlof an insect cellular promoter and an enhancer in substantially alltissues of the insect.

An enhanced expression cassette may be constructed which contains aninsect cellular promoter, an enhancer and a termination sequence. Thecassette also contains a polylinker sequence comprising a number ofunique restriction sites into which the desired structural gene encodinga heterologous protein may be inserted. Preferably the enhancedexpression cassette comprises the plasmid pBmeA. The desiredheterologous gene may be inserted into the expression cassette such thatthe structural gene is functionally linked to the insect cellularpromoter and the enhancer.

Another vector may also be constructed which contains a portion of thebaculovirus that can sustain insertions of non-viral DNA fragments. Therecombinant enhanced expression cassette containing the heterologousgene may be excised from the first vector and inserted into a nucleotidesite in the baculovirus sequence present on the other vector, therebycreating a transplacement vector. The recombinant expression cassette orthe transplacement vector may each be transfected into insect cells forexpression of the heterologous protein. The transplacement vector mayalso be co-transfected into insect tissue culture cells with wild-typebaculovirus DNA which is comprises a DNA fragment homologous to theportion of baculovirus DNA present on the transplacement vector.Recombinant baculovirus genomes containing the expression cassette maythen be generated by double cross-over events. Such recombinant virusescan be used to infect insect cells for the purposes of incapacitatingthe insects.

If a continuous high level expression of foreign genes is desired ininsect lines, cells may be transformed with enhanced recombinantexpression cassettes employing cellular promoters and enhancersfunctionally linked to the foreign gene. To achieve continuous highlevel of expression of foreign genes, in one embodiment, normal insecttissue culture cells can be transformed with a plasmid containing anenhanced expression cassette comprising a cellular promoter and anenhancer functionally linked to a heterologous gene and an extra geneexpressing a selective marker (e.g. antibiotic resistance gene under thecontrol of a promoter that functions constitutively in insect cells).Application of a relevant selection should lead to integration of one ormore multiple copies of the plasmid into the chromosomes of the cells,thus generating an insect cell line capable of continuous high levelexpression of the foreign gene present in the recombinant expressioncassette. The level of transcription from the cellular promoterfunctionally linked to an enhancer as compared to the level oftranscription from the cellular promoter alone is preferably at leastabout 10 fold and more preferably at least about 100 fold.

The selective marker gene can be any gene which codes for a protein,wherein cells producing this protein can be positively selected from thegeneral population. The type of selectable marker is not critical solong as cells having the marker may be selected. Selectable markersuseful in the present invention are antibiotic resistance genes, forexample the neomycin gene, the hygromycin gene and the dihydrofolatereductase gene (DHFR).

In another embodiment the insect cells can be transformed with a vectorcontaining the IE-1 gene and a suitable resistance/selectable markergene. Application of a relevant selection should lead to integration ofone or more multiple copies of the vector into the chromosomes of thecells, thus generating an insect cell line capable of continuous highlevel expression of the IE-1 gene product. Thus the cell line willcontain the IE-1 gene in the absence of added baculovirus. Such a cellline can be subsequently transformed with additional vectors containingeither the recombinant expression cassette containing an insect cellularpromoter functionally linked to a heterologous gene or the enhancedrecombinant expression cassette containing an enhancer and an insectcellular promoter functionally linked to a heterologous gene. The methodof creating expression vectors containing insect cellular promoters isdescribed in U.S. patent application Ser. No. 07/904,408 (now abandoned)which is incorporated by reference herein in its entirety. The secondvector may also comprise an additional gene conferring resistance to asecond selection agent. In both cases, synthesis of the foreign proteinwill be continuous, because integrated expression cassettes cannot belost through replication and the insect cells never die because they arenot infected by any viruses. Expression will also occur at a high level.The level of production of heterologous proteins in cells expressing theIE-1 gene as compared to cells without the IE-1 gene is preferably atleast about 10 fold greater and more preferably at least about 100 foldgreater. The production of heterologous proteins in cells expressing theIE-1 gene where the heterologous gene is functionally linked to anenhancer is preferably at least about 100 fold greater and morepreferably at least about 1000 fold greater than in cells lacking boththe IE-1 protein and the enhancer.

In one embodiment, the desired heterologous protein is thechloramphenicol acetyltransferase structural gene sequence (CAT). Thisgene, cat, was inserted into the BamHI site of pBmA to create pBmA.cat.In another embodiment a fragment containing the sequence for Heliothisvirescens juvenile hormone esterase cDNA (Hanzlik et al., 1990) isinserted into the EcoRI site of the polylinker of plasmid pBmA. Inanother embodiment a fragment containing the sequence for bovinepreproenkephalin cDNA (Gubler and Hoffman, 1983) is inserted between theEcoRI and BamHI sites of the polylinker of plasmid pBmA. In anotherembodiment a fragment containing the gene for Drosophila FMRFamide-likepeptide precursor (Schneider and Taghert, 1990) is ligated into theEcoRI site of the polylinker of plasmid pBmA. In other embodiments thegenes for the neuropeptide proctolin, mouse tissue non-specific alkalinephosphatase, chicken protamine gene, Pyemotes tritici insectotoxin PxP-1and Androctonus australis insectotoxin AaIT will also be inserted intothe expression cassette.

If the development of recombinant baculoviruses for use as insect pestcontrol agents is desired, the enhancer is inserted into the expressioncassette containing the insect cellular promoter. An incapacitating geneins then placed under the control of the insect cellular promoter andthe enhancer. A recombinant baculovirus in the form of an occlusion bodyis created to contain the enhanced recombinant expression cassette bythe methods of this invention. Insects may then be infected with therecombinant baculovirus in the form of an occlusion body. In the case ofrecombinant baculoviruses a plasmid containing the IE-1 gene is notrequired since the IE-1 gene is produced in the infected cells by thebaculovirus' genome immediately upon infection. In the case ofrecombinant baculoviruses, the level of enhancement, relative torecombinant viruses without enhancers will preferably be at least about10 fold, more preferably at least about 100 fold.

Using the methods described in this invention, a person skilled in theart can construct an recombinant expression cassette containing aninsect cellular promoter and an enhancer able to direct the expressionof any desired heterologous protein such that when the recombinantexpression cassette is introduced into insect cells the heterologousprotein will be expressed at an increased level in substantially all ofthe insect tissues at an early stage in the infection. It is alsoobvious that the invention can be applied to any available insects thatare subject to baculovirus infection.

The following examples are offered to illustrate the present inventionand should not be construed in any way as limiting the scope of thisinvention.

EXAMPLES

Chemicals used in the following examples were obtained from thefollowing companies:

Amersham Canada Ltd., Oakville, Ontario, Canada

J.T. Baker, Phillipsburg, N.J.

BioRad Laboratories Ltd. Canada, Mississauga, Ontario, Canada

Boehringer Mannheim, Laval, Quebec, Canada

GIBCO BRL Canada, Burlington, Ontario, Canada

Hyclone Laboratories, Inc., Logan, Utah

JRH Biosciences, Inc., Lenexa, Kans.

New England Biolabs, Inc., Mississauga, Ontario, Canada

Pharmacia LKB, Baie d' Urfe', Quebec, Canada

Promega Corporation, Madison, Wis.

Sigma, St. Louis, Mo.

Stratagene, La Jolla, Calif.

United States Biochemicals, Cleveland, Ohio

All enzymes used for the construction and characterization of therecombinant plasmids and baculoviruses were obtained from Pharmacia,LKB; New England Biolabs, Inc.; GIBCO BRL Canada; Boehinger Mannheim;and used according to those suppliers recommendations. The cloningprocedures set forth in the examples are standard methods described inSambrook et al., Molecular Cloning: A Laboratory Manual, Cold SpringHarbor Laboratory (1982) which is incorporated herein by reference. Thisreference includes procedures for the following standard methods:cloning procedures with E. coli plasmids, transformation of E. colicells; plasmid DNA purification, agarose gel electrophoresis,restriction endonuclease digestion, ligation of DNA fragments and otherDNA-modifying enzyme reactions.

Example 1 Plasmid Constructions

The vector, pBmA (FIG. 1A), is a pBluescript (Stratagene) derivative ofclone pA3-5500 which contains the A3 cytoplasmic actin gene of Bombyxmori (Mounier and Prudhomme, 1986). Plasmid pBmA was constructed tocontain 1.5 kb of the A3 gene 5′ flanking sequences and part of itsfirst exon to position +67 (relative to transcription initiation), apolylinker region derived from plasmid pBluescript (Stratagene) forinsertion of foreign gene sequences, and an additional 1.05 kb of the A3gene sequences encompassing part of the third exon of the gene fromposition +836 and adjacent 3′ flanking sequences which contain signalsrequired for RNA transcript polyadenylation. This expression vector wasconstructed by (1) subcloning into plasmid Bluescript SK+ (Stratagene) a1.5 kb Kpnl/AccI fragment of clone pA3-5500 containing the 5′ flanking,5′ untranslated and coding sequences of the A3 gene up to position +67to generate plasmid pBmAp; (2) mutagenizing the ATG translationinitiation codon present at position +36 to +38 of the actin codingsequence in plasmid pBmAp into AGG, AAG or ACG by the method of Kunkel(1985) to generate plasmids pBmAp.AGG, pBmAp.AAG and pBmAp.ACG; (3)subcloning into plasmid pSP72 (Promega Corporation) a 1.05 kb XhoI/SaIIfragment of clone pA3-5500, containing part of the third exon of theactin gene from position +836 and adjacent 3′ flanking sequences whichinclude signals required for RNA transcript polyadenylation, to generateplasmid pBmAt; (4) converting the unique XhoI site of plasmid pBmAt intoa NotI site by digestion of this plasmid with XhoI (GIBCO BRL),end-filling with Klenow DNA polymerase (Boehringer Mannheim), ligationof NotI linkers (DNA Synthesis Laboratory, University of Calgary) withDNA ligase (New England Biolabs, Inc.), digestion with NotI (New EnglandBiolabs, Inc.) and religation with T4 DNA ligase (Boehringer Mannheim)to generate plasmid pBmAtN; (5) isolating the actin insert fragment fromplasmid pBmAtN by double digestion with NotI and SacII (New EnglandBiolabs, Inc.) and electroelution following separation on an agarosegel; (6) ligating the NotI/SacII actin fragment from (5) above intoNotI/SacII-digested plasmids pBmAp.AGG, pBmAp.AAG and pBmAp.ACG with T4DNA ligase to generate the actin expression cassettes pBmAo.AGG,pBmAo.AAG and pBmAo.ACG. The actin expression cassette pBmA (FIG. 1A)was derived from plasmid pBmAo.AGG by digestion with SaII (completedigestion) and BamHI (partial digestion) to remove part of thepolylinker sequence of plasmid pSP72 present at the 3′ terminus of theactin insert, and religation with T4 DNA ligase. Translation initiationfrom any gene inserted in the pBmA actin expression cassette polylinker(FIG. 1A) occurs from the first ATG triplet of the insert.

A 900 bp XhoII fragment containing the chloramphenicol acetyltransferase (CAT) open-reading frame was excised from pCARCAT-1(Mitsialis et al., 1987) by digestion of the DNA with the restrictionenzyme XhoII. This fragment includes the entire coding information ofCAT, all of the 5′ and most of the 3′-untranslated gene sequences, aswell as 63 bp of the 5′ untranslated region of the early transcriptionunit of SV40. It should be emphasized that the SV40 sequences arecompletely devoid of any promoter or enhancer elements. Therefore, thetranscripts must depend on control elements contributed by the actinpromoter sequences. This fragment was inserted into the BamHI site ofpBmA to create pBmA.cat (FIG. 1B).

To produce an appropriate viral transplacement vector plasmid pBmp2,which contains a copy of the BmNPV polyhedron gene, whose promoter andpart of the coding sequences have been deleted, was created (Iatrou andMeidinger, 1989). Initially plasmid Bmp/pP3 was constructed as describedin Iatrou et al. (1985). More specifically, the B. mori baculovirusgenome was digested with PstI, a 10 kb PstI fragment containing thepolyhedron gene was identified by Southern blot hybridization using³²P-labelled polyhedron gene sequences of AcNPV, purified byelectroelution following agarose gel electrophoresis and cloned into thePstI site of pUC9. Bal 31 deletion mutagenesis was accomplished bylinearizing plasmid Bmp/pP3 DNA at a unique XbaI site located 194 bpdownstream of the 5′ terminus of the polyhedron gene (nt 147 of theprotein coding sequence between codons 49 and 50; see Iatrou et al.(1985) for sequence details) and incubating with exonuclease Bal 31 in areaction containing 0.6 M NaCl, 12.5 mM CaCl₂, 12.5 mM MgCl₂, 20 mMTris-HCl, pH 7.8, 160 μg/ml linearized plasmid DNA and 75 units/ml ofBal 31 (New England Biolabs, Inc.) at 30° C. Aliquots were withdrawn at10 min. and at 3 min. intervals thereafter, up to a total of 25 minutes,and flushed into a tube containing two reaction volumes of 50 mM EDTAand two volumes of water-saturated phenol. Following purification byphenol extraction, combined aliquots were 3′ end-filled with Klenow DNApolymerase, as described (Iatrou et al., 1985). XbaI linkers wereligated to the end-filled DNA in a reaction containing 66 mM Tris-HCl,pH 7.5, 66 μM ATP, 6.6 mM MgCl₂, 10 mM DTT, 0.125 μg/ml of XbaI linkers,4 μg/ml of end-filled DNA and 300 units/ml T4 DNA ligase (PharmaciaLKB). Linker-ligated DNA was purified by electroelution, restricted withXbaI, and circularized by incubating in the above reaction mix at a DNAconcentration of 0.4 μg/ml with 30 units/ml T4 DNA ligase.Transformation of circularized plasmid DNA into Escherichia coli HB101resulted in the generation of a library of clones containing polyhedrongenes with segmental deletions of various lengths starting from theunique XbaI site of the gene. Plasmid DNA from several clones wascharacterized by sequence analysis (Maxam and Gilbert, 1977) and plasmidpBmp2 which has the region from nt-90 to nt 339 of the polyhedron genedeleted was selected as the recipient for insertion of the actinexpression cassette.

Plasmid pBmp2 was digested with XbaI and the XbaI site converted to anSstI site by the addition of linkers and religation. The resultingplasmid was designated pBmp2s (FIG. 1C).

The SstI fragment containing the A3 promoter-CAT structural gene DNAfragment was removed from pBmA.cat by digestion with SstI and theninserted into the SstI site of pBmp2s to create pBmp2s/A.cat (FIG. 1D).A recombinant virus, BmNPV/A.cat was obtained using this transfer vectoras described in Example 2.

Plasmid pBmp26.cat (FIG. 1E) contains a mutated polyhedron gene promoterdirecting the expression of the cat structural gene. It has beenconstructed by subcloning the cat structural gene into plasmid pBmp26T.Plasmid pBmp26T is a derivative of plasmid pBmp26 which was initiallyselected from the library of clones generated by Bal 31 deletionmutagenesis of the polyhedron gene described above for plasmid pBmp2,and shown by DNA sequence analysis to encompass a deletion ofnucleotides +27 to +251 (relative to translation initiation) of thepolyhedron gene. The polyhedron gene translation initiation codon ATGwas removed from plasmid pBmp26 containing the deleted polyhedron gene,on a 2.0 kb XhoI/XbaI fragment and mutationally converted into ATT bythe method of Kunkel (1985). The mutated 2.0 kb fragment was then clonedinto XhoI/XbaI digested Bmp/pP14 to create pBmp26T. Plasmid Bmp/pP14 wascreated in the same manner as Bmp/pP3 described above (Iatrou et al.,1985), but contains the baculoviral sequences in the oppositeorientation. As a result, pBmp26T has a deletion from +27 to +146 withATT instead of ATG. Plasmid pBmp26T was then digested with XbaI and the900 bp XhoII CAT structural gene open-reading frame was inserted intothe plasmid by blunt end ligation to create pBmp26.cat. The polyhedronpromoter sequences are retained in this plasmid and direct thetranscription of the cat structural gene.

Example 2 Cell Culture and Viruses

Bombyx mori Bm5 silkworm tissue culture cells (Grace, 1967) weremaintained in IPL-41 medium (JRH Biosciences, Inc.) containing 10% fetalcalf serum (Hyclone Laboratories, Inc.), as previously described (Iatrouet al., 1985). Recombinant viruses were obtained by co-transfection ofBm5 cells with pBmp2s/A.cat DNA or pBmp26.cat DNA and wild-type BmNPVDNA. Bm5 cells were transfected in 6-well microtiter plates in a mannersimilar to that described by Iatrou and Meidinger (1989). Typically, Bm5cells were plated in 10 cm² plates or microtiter wells at a density of2×10⁵ cells/cm². Transfection was accomplished by removing the culturemedium, rinsing the cell monolayer with basal medium (no fetal calfserum) and adding 500 μl of transfection solution [30 μg/ml Lipofectin(GIBCO BRL Canada) in basal IPL-41 (JRH Biosciences, Inc.) containing 5μg/ml of the transfer vector DNA and 0.2 μg/ml BmNPV DNA]. Afterincubation for 5 hours, the transfection solution was removed, the cellsrinsed with basal medium and 2 ml of complete medium containing 50 μg/mlof gentamicin was added. A purified recombinant virus was obtained byserial dilution as described previously (Pen et al., 1989; Goswami andGlazer, 1991) and its structure confirmed by Southern hybridization with³²P-labelled 900 bp XhoII cat gene fragment and nucleotide sequenceanalysis (Maxam and Gilbert, 1977).

The purified virus was propagated by infecting Bm5 cells grown in 25 cm²flasks at a density of 1×10⁶ cells/ml. The medium from the infectedcells, containing recombinant virus, was collected 4 to 7 days postinfection and used as inoculum for subsequent experiments.

Example 3 Infections

A. Bm5 cells to be infected with virus were seeded into 24-wellmicroliter plates at a density of 2×10⁵ cells (in 500 μl medium) perwell. After 3 days, 100 μl of viral inoculum [10⁶ plaque forming units(pfu)] obtained by the method of Example 2, was added to each well. Timeafter infection was counted from the time that the virus was added tothe medium. Cells were removed from the wells at the appropriate time byrepeated pipeting.

Cells were pelleted from the medium at 3000×g for 5 min, suspended in 1ml PBS (10 mM KH₂PO₄, 2 mM NaH₂PO₄, 140 mM NaCl, 40 mM KCl) andrepelleted. The cells were then resuspended in 200 μl of 0.25 MTris-HCl, pH 7.8, freeze-thawed three times using dry ice to disrupt thecells and, after centrifugation, the supernatants retained for CATassays.

B. Silkworm larvae were reared on a diet of fresh mulberry leaves, andwere injected with virus at the beginning of the 5th instar. Afterincubating the animals at 0° C. for 1 hour, 10 μl of viral inoculum (10⁵pfu) obtained by the method of Example 2 was injected into the haemocoelusing a 26 gauge needle. The infected animals were maintained as beforeand collected at the appropriate time for dissection.

Larvae were dissected in cold PBS and the appropriate tissues wereremoved. Midguts were cut open longitudinally to allow removal of thegut contents and all tissues were rinsed extensively with severalchanges of PBS. The tissue samples were ground in 0.25 M Tris-HCl, pH7.8 with a small pestle in a microcentrifuge tube, freeze-thawed andcentrifuged as described about. All larval extracts were heated to 65°C. for 5 minutes to inactivate cellular deacetylase activities beforeuse in CAT assays.

Example 4 Protein and CAT Assays

Assays for protein content and CAT activity of the extracts fromtransfected and infected cells and infected larvae were performed asfollows. For CAT assays, cells (usually 2×10⁵ to 2×10⁶ on plating day)were collected 24 hours post-transfection or 48 hours post-infection andrinsed with PBS (10 mM KH₂PO₄, 2 mM NaH₂PO₄, 140 mM NaCl, and 40 mMKCl). After pelleting at 3000×g for 5 minutes, the cells wereresuspended in 100 μl of 0.25 mM Tris-HCl, pH 7.8 and the suspension wasfreeze-thawed in dry ice 3 times to disrupt the cells. Aftercentrifugation, the protein content of the supernatant was determined bythe Bradford protein assay (Bradford, 1976) using BioRad LaboratoriesLtd. Canada protein assay reagent and bovine serum albumin as thestandard. Samples were assayed for CAT activity (Gorman et al., 1982) in150 μl total reaction volumes containing 0.25 M Tris-HCl, pH 7.8, 0.5 mMacetyl-CoA (Sigma), 2.4 nmol [¹⁴C]-chloramphenicol (50 mCi/nmol;Amersham Canada Ltd.). After 1 hour of incubation at 37° C., thereactions were extracted with 500 μl of ethyl acetate and dried. Theresulting residues were resuspended in 15 μl of ethyl acetate andspotted onto silica-gel thin-layer chromatography plates (J.T. Baker).The plates were developed for 2 hours in a 95:5 chloroform-methanolsolvent mixture, after which they were dried and autoradiographed. Insome instances, the areas of the reaction products (the acetylated formsand the residual starting material) were cut out of the plates andquantitated by liquid scintillation counting. One unit of CAT activitycatalyzes the acetylation of one nanomole of chloramphenicol per minuteat 37° C.

Example 5 Presence of CAT Activity in Recombinant Viruses

It was necessary to determine whether the infected cells would containany background level of CAT activity as a result of the extremely stableCAT protein being encapsulated within the recombinant non-occluded virus(NOV) and released within the cell immediately upon infection.Non-occluded virus (NOV) was isolated from the medium by centrifugationat 50,000 rpm in a Beckman 100.2 rotor for 1 hour. After removal of thesupernatant, the viral pellet was rinsed with 1 ml H₂O, resuspended in1.6 ml H₂O and recentrifuged as before. The final pellet was againrinsed and resuspended in H₂O. 20 μl of isolated non-occluded virus ormedium containing NOV was assayed for 1 hour as described in Example 4.The results are shown in FIG. 2. The upper spot (Ac-C) represent acetylchloramphenicol, the products of the CAT reaction, while the lower spotin each assay (C) represents the substrate, chloramphenicol. Medium,collected from infected cells 5 days after infection and used asinoculum, was found to contain high levels of CAT activity (FIG. 2A,right). Non-occluded virus (NOV) isolated from the medium was lysed bysonication and was also found to contain a significant amount of CATactivity even after extensive washing (FIG. 2A, left).

Bm5 cells were infected with BmNPV/A.cat or BmNPV/P26.cat inoculumobtained by the method of Example 2. The cells were assayed for CATactivity by the method of Example 4. Cells infected with eitherrecombinant virus were found to contain background CAT activity 5minutes after addition of the viral inoculum. The level of CAT activityin these cells reached a plateau within 20 minutes (FIG. 2B) and thislevel was maintained for several hours.

Bm5 cells were infected with BmNPV/A.cat or BmNPV/P26.cat inoculum ofExample 2 containing medium and virus, or with NOV isolated by themethod described in this example or were mock inoculated. The cells werecollected 1 hour after infection and were washed once (1×) or five (5×)times with 1 ml PBS. When cells were infected with the standardinoculum, which contained both virus and medium, a portion of this CATactivity could be removed by washing the cells with PBS, but asignificant fraction remained even after extensive washing. When cellswere infected with isolated virus, none of the activity could be removedby washing (FIG. 2C). It was concluded, therefore, that this remainingactivity represented CAT enzyme attached to or introduced into the cellsduring viral infection, and that this background value would have to besubtracted from all subsequent measurements in order to quantitate theamount of CAT enzyme present due to promoter activity during time courseexperiments.

Example 6 Time Course of Gene Expression in Bm5 Cells

Bm5 cells were infected with recombinant virus BmNPV/A.cat orBmNPV/P26.cat by the methods of Example 3 and cell extracts were assayedfor CAT activity at various times after infection. The CAT assays wereperformed using 5 μg cell protein incubated for 1 hour from cellsinfected with BmNPV/A.cat and 1 μg cell protein for 1 hour from cellsinfected with BmNPV/P26.cat by the method described in Example 4.

CAT activity above the background level, determined by the method ofExample 5, was first detected in cells infected with BmNPV/A.cat at 5hours post-infection (“p.i.”) (FIG. 3A), while in cells infected withBmNPV/P26.cat, activity above background was not observed until the 20hour time point (FIG. 3B). The cell extracts were then dilutedappropriately to obtain quantitative CAT assays for all time points andthe background values, determined by the method of Example 5 at 1-2hours post-infection, were subtracted from each point to generate thecurves shown in FIG. 3C. At early times, from 5-12 hours p.i., the actinpromoter was transcriptionally active resulting in significant CATactivity above background level, while no activity could be detectedfrom the polyhedron promoter. CAT activity derived from the polyhedronpromoter was detected at 20 hours p.i. At 20 hours p.i, the actinpromoter was more active than the polyhedron promoter, while at 30 hoursp.i. CAT activity from the two promoters was roughly equal. Finally,after 30 hours p.i. expression from the polyhedron promoter was higherthan that from the actin promoter (by a factor of 3 at 50 hours p.i.,the last point of the time course). Therefore, in Bm5 cells the actinpromoter was active 15 hours earlier than the polyhedron promoter. It islikely that the actin promoter is active immediately upon insertion ofthe viral genome into the cells.

Example 7 Expression in Infected Larvae

Fifth instar B. mori larvae were injected with recombinant virusBmNPV/A.cat or BmNPV/P26.cat by the method of Example 3 and varioustissues were initially assayed for CAT activity 2 days post-infection bythe method of Example 4 (FIG. 4). Tissues from three larvae were pooledfor each assay. Larvae infected with either virus contained high levelsof CAT activity in both the head and body wall, a lower level in themidgut, and still lower expression in the gonads. Although detectable,only very low levels of CAT activity could be seen in the silk glands.

In a second set of experiments, larvae at the beginning of fifth instarwere injected with the recombinant viruses BmNPV/A.cat or BmNPV/P26.catby the method of Example 3 and body wall tissues were collected atdifferent times post infection. The samples were then assayed for CATactivity by the method of Example 4 (FIG. 5A, B and C).

Tissues from the injected larvae contained a low level of background CATactivity at early times post infection. In larvae infected withBmNPV/A.cat, CAT activity above background was first observed in bodywall tissues at 24 hours p.i. (FIG. 5A). Larvae infected withBmNPV/P26.cat did not express any CAT activity above background levelsuntil 48 hours p.i. (FIG. 5B). A quantitative analysis of the resultsobtained from all time points following subtraction of the backgroundvalues is shown in FIG. 5C. Even at 48 hours p.i. the actin promoter wassignificantly more active than the polyhedron promoter. At 60 hours p.i.the activity from the polyhedron promoter was found to be higher thanthat from actin but the difference in expression levels at that pointwas only six-fold.

Similar results were obtained from the other tissues that were examined.In all these experiments, activity from the actin promoter was seen 24hours earlier than from the polyhedron promoter. Although the times atwhich CAT activity was first expressed from the two viruses in infectedlarvae was later than that observed in infected Bm5 cells, the generalpattern of expression in vivo was similar to that observed in vitro.

Example 8 Expression in Other Lepidopteran Species

To determine the level of activity of the B. mori cytoplasmic actinpromoter in cells of other insect species, plasmid pBmA.cat wastransfected into tissue culture cells of both B. mori (Bm5) andSpodoptera frugiperda (Sf21) in a 24 well microliter plate. Wells wereseeded with either 2×10⁵ Bm5 cells or 4×10⁵ Sf21 cells and the cellsincubated in 200 μl transfection solution containing 5 μg/ml of pBmA.catby the method of Example 2. Two days after transfection, cells from eachwell were collected, washed with PBS, lysed into 200 μl 0.25 M Tris-HCl,pH 7.8 and CAT assays were performed using either 5 μl or 25 μl of cellextracts by the method of Example 4. The Bm5 cells contained about 4times as much CAT activity as the equivalent volume of Sf21 cells (FIG.6A).

Transfected cells were collected, washed as above and suspended in 200μl of PBS. The cell suspensions (2 μl or 5 μl) were dot blotted ontoHybond N+ membrane (Amersham, Canada Ltd.) and treated with 0.5 M NaOHfollowed by 0.5 Tris-HCl, pH 7.5. Labeling of linearized pBS/SK+ DNAwith α-³²P-dCTP and hybridization of this probe to the membrane wascarried out at 65° C. as described by Fotaki and Iatrou (1988) J. Mol.Biol., 203:849-860. After hybridization, the membrane was washed at 65°C. in 0.1×SSC (1×SSC is 0.15 M Nacl, 0.015 M Nacitrate, pH 7.0), 0.1%SDS and exposed to X-ray film. DNA hybridization indicated that, underthe transfection conditions used, the Bm5 cells had taken up at leasttwice as much plasmid DNA as equivalent volumes of Sf21 cells (FIG. 6B).

These experiments, therefore, indicate that the B. mori actin promoteris active in S. frugiperda cells at levels nearly equivalent, if notequal, to those observed in B. mori cells. Similar experiments in whichpBmA.cat was transfected into Cf124 tissue culture cells of the sprucebudworm, Choristoneura fumiferana, have shown that the actin promoter isactive in the latter cells at levels nearly identical to those observedin B. mori cells.

Example 9 Oral Feeding

The occluded form of BmNPV/A.cat was produced by infecting Bm5 cellssimultaneously with the non-occluded forms of wild type BmNPV andBmNPV/A.cat. and collecting the resultant occlusion bodies by thefollowing method. Tissue culture flasks (25 cm²) were seeded with 2×10⁷Bm5 cells in 25 ml of IPL-41 medium. After 24 hours, 2×10⁶ plaqueforming units (pfu) each of wild type and recombinant extracellularvirus were added to the cells. At five days post-infection (p.i.) cellsand polyhedra were collected by centrifugation at 10,000×g for 5minutes. The supernatant was removed and 5 ml of 0.4% SDS, 10 mMTris-HCl pH 7.8 was added to the pellet from each flask. Cells wereincubated for 2 hours at 22° C. with occasional agitation and polyhedrawere precipitated by centrifugation at 10,000×g for 10 minutes.Polyhedra were then suspended in H₂O to the desired concentration, andstored at −70° C. in small aliquots.

Polyhedron derived viruses (PDV) were prepared from the polyhedra bysuspending 1.2×10⁶ polyhedra in 20 μl of freshly prepared 50 mM NaCl, 6mM Na₂CO₃. After incubation at 22° C. for 1 hour with occasionalagitation, the samples were spun at 12,000 rpm at 4° C. in amicrocentrifuge for 1 hour to pellet PDV. These pellets were resuspendedin 50 μl H₂O and centrifuged as before. Pellets were then suspended in 1ml IPL-41 medium.

Cells to be infected with PDV were seeded into wells of a 6-wellmicrotiter plate (10⁶ cells in 1 ml medium) and 1 ml of the mediumcontaining PDV was added. At 6 days p.i., the cells were collected andDNA isolated as previously described in Iatrou et al. (1985). DNA (2 μg)from these infected cells (a mixture of cellular and viral DNA) wasdigested with HinDII and separated on a 1% agarose gel. After transferto nylon membranes, the DNA was probed with a 2.6 kb HinDII fragment ofthe BmNPV genome including the polyhedron gene as previously describedin Example 8.

Each of the pure viruses has a distinct pattern on Southern blottingwhich can be used to determine the composition of a mixed population(FIG. 7). Analysis of cells infected with PDV from these “mixed”polyhedra indicated that these polyhedra contained a mixture of PDV fromthe wild type virus and from the recombinant virus.

Bombyx mori larvae were reared on a diet of fresh mulberry leaves andwere infected with the virus at the beginning of the third instar.Larvae were starved for 6 hours at 25° C. and then placed at 4° C.overnight. After incubation at 22° C. for 1 hour, each larvae was givena 1×2 cm piece of mulberry leaf onto which a suspension of polyhedronhad been spread (approx. 3×10⁴ polyhedra). After a period of 6 hours, inwhich most of the infected leaf material was consumed, larvae wereallowed to feed on fresh uninfected leaves. Larvae were dissected andCAT assays were performed as previously described in Example 4. Extractsfrom ten larvae were pooled for each assay. See FIG. 8.

CAT activity was observed at earlier times in the larvae infected withviruses containing the CAT gene under the control of the actin promoterthan was observed when the larvae were infected with viruses containingthe CAT gene under the control of the polyhedron promoter. In larvaeorally infected with a mixture of BmNPV/A.cat and BmNPV a small amountof CAT activity could be detected in the gut at 7 hour p.i. and greateramounts at 12 and 24 hours p.i. In the body wall, significant CATactivity was present by 24 hours p.i. In larvae orally infected with amixture of BmNPV/P26.cat and BmNPV, no CAT activity could be detected inthe gut until 24 hours p.i. while in the body wall, no CAT activity hadyet accumulated at that time. (FIG. 8) The time course of activity ofthe actin and polyhedron promoters in orally infected larvae is thussimilar to that observed in larvae infected by hemocelic injection withextracellular virus.

The following examples are provided to illustrate the generalapplicability of the concept of utilization of recombinant baculovirusescontaining cellular promoter-based recombinant expression cassettesdirecting early expression of heterologous genes (proteins) in infectedinsects.

Example 10 Early Expression of Additional Heterologous Genes under theControl of the Actin Promoter of B. mori in Insect larvae Infected withRecombinant BmNPVs

Recombinant BmNPVs expressing the heterologous proteins juvenile hormoneesterase (BmNPV/A.jhe), enkephalin-like peptides (BmNPV/A.enk),proctolin (BmNPV/A.pro), alkaline phosphatase (BmNPV/A.aph), protamine(BmNPV/A.cpr), FMRF-like peptides (BmNPV/A.fmrf), insectotoxin TxP-I(BmNPV/A.TxP-I) and insectotoxin AaIT (BmNPV/A.AaIT) are constructed.Each of these recombinant baculoviruses incorporates a recombinant B.mori actin expression cassette containing a fragment of DNA encoding thecorresponding heterologous protein, such that expression of theheterologous protein in insect cells infected with the recombinantbaculovirus is directed by the B. mori actin promoter. The methoddescribed in Example 1 is employed to generate the recombinantexpression cassettes described below, and the latter are used inconjunction with plasmid pBmp2s (FIG. 1C) to generate the correspondingtransplacement vectors as described in Example 1.

(1) pBmA.jhe: A 3.0 kb EcoRI fragment from plasmid 3hv16 containing thesequence for Heliothis virescens juvenile hormone esterase cDNA (Hanzliket al., 1990) is isolated by digestion of 3hv16 with EcoRI and insertedinto the EcoRI site of the polylinker of plasmid pBmA (FIG. 1A).

(2) pBmA.enk: A 1.2 kb NcoI/BamHI fragment excised from plasmid 921containing the sequence for bovine preproenkephalin cDNA (Gubler andHoffman, 1983) is isolated by digestion of plasmid 921 with NcoI and the1.2 kb fragment is inserted between the EcoRI and BamHI sites of thepolylinker of plasmid pBmA.

(3) pBmA.pro: Two fragments of synthetic DNA encoding the amino acidsequences WPKRRRYLPTXRPEW (SEQ ID NO:3) and WPKRRYLPTKRPEW (SEQ IDNO:5), respectively, which include the amino acid sequence of theneuropeptide proctolin, RYLPT (Starrat & Brown, 1975) are separatelyinserted into the unique BstXI site of plasmid pBmA.enk. The nucleotidesequences of the two synthetic fragments are:

     1.(SEQ ID NO:2)5′     GTGGCCAAAGAGAAGAAGATACCTCCCCACCAAGAGACCAGAGTG 3′3′TCACCACCGGTTTCTCTTCTTCTATGGAGGGGTGGTTCTCTGGTC 5′      2.(SEQ ID NO:4)5′    GTGGCCAAAGAGAAGATACCTCCCCACCAAGAGACCAGAGTG 3′3′TCACCACCGGTTTCTCTTCTATGGAGGGGTGGTTCTCTGGTC 5′

The sequences of the protruding 3′ termini of the two syntheticfragments above allow their unidirectional insertion into the BstxI siteof pBmA.enk. The resultant insertions are in-frame with the nucleotidesequences of preproenkephalin CDNA present in pBmA.enk such thattranslation of the mRNA synthesized under the control of the actinpromoter yields fusion proteins which are longer than preproenkephalinby 15 or 14 amino acids, respectively, and contain the proctolin peptidesequence, RYLPT.

(4) pBmA.aph: A 2.4 kb EcoRI fragment from plasmid p.1.111 containingthe structural gene for mouse tissue non-specific alkaline phosphastase(Hahnel and Schultz, 1989) is isolated by digestion of p1.111 with EcoRIand the 2.4 kb fragment is ligated into the EcoRI site of the polylinkerof plasmid pBmA.

(5) pBmA.fmrf: A 1.4 kb EcoRI fragment from plasmid pHS2A-3 containingthe gene for Drosophila FMRFamide-like peptide precursor (Schneider andTaghert, 1990) is isolated by digestion of pH52A-3 with EcoRI and the1.4 kb fragment is ligated into the EcoRI site of the polylinker ofplasmid pBmA.

(6) pBmA.cpr: A 0.45 kb Smal fragment from plasmid CPC4S4 containing thechicken protamine gene (Oliva and Dixon, 1989) is isolated by digestionof CPC454 with SmaI and the 0.45 kb fragment is ligated into the Smalsite of the polylinker of plasmid pBmA.

(7) pBmA.TxP-1: A 0.94 kb EcoRI fragment from plasmid pTox-34 containingthe sequence for Pyemotes tritici insectotoxin TxP-1 cDNA (Tomalski andMiller, 1991) is isolated by digestion of pTox-34 with EcoRI and the0.94 kb fragment is ligated into the EcoRI site of the polylinker ofplasmid pBmA.

(8) pBmA.AaIT: A 0.3 kb BamHI fragment from plasmid pTZ-AaIT containingthe sequence for Androctonus australis insectotoxin AaIT cDNA (McCutchenet al., 1991) is isolated by digestion with BamHI and the 0.3 kbfragment is ligated into the BamHI site of the polylinker of plasmidpBmA.

Transplacement vectors pBmp2s/A.jhe, pBmp2s/A.enk, pBmp2s/A.aph andpBmp2s/A.fmrf pBmp2s/A.pro, pBmp2s/A.cpr, pBmp2s/A.TxP-I andpBmp2s/A.AaIT will be generated from the recombinant expressioncassettes listed above, in conjunction with plasmid pBmp2s (FIG. 1C), asdescribed in Example 1 for transplacement vector pBmp2s/A.cat (FIG. 1D).Non occluded recombinant baculoviruses BmNPV/A.jhe, BmNPV/A.enk,BmNPV/A.pro, BmNPV/A.aph, BmNPV/A.fmrf, BmNPV/A.cpr, BmNPV/A.TxP-I andBmNPV/A.AaIT will be generated by co-transfection of Bm5 cells with DNAfrom wild-type BmNPV and each of the above transplacement vectors,purified and amplified, as described in Example 2.

Infection of insect larvae with each recombinant virus, as described inExample 3, will result in expression of the respective heterologousproteins under the control of the actin promoter of B. mori inessentially all insect tissues at an early stage of introduction,similar to that observed with BmNPV/A.cat. Assessments of timing ofexpression will be based on hybridizations of RNA extracted from tissuesof infected larvae to ³²P-labelled probes derived from the heterologousgenes; polymerase chain reaction (PCR) amplification of cDNA generatedform the same RNA using appropriate primers derived from the knownsequences of the heterologous genes; detection of the heterologousproteins themselves using available antibodies; and, in the cases ofBmNPV/A.TxP-1 and BmNPV/A.AaIT, also by monitoring the behavior of theinfected larvae for paralytic symptoms. Oral infection of insect larvaewith occluded forms of the same recombinant baculoviruses, generated asdescribed in Example 9 for BmNPV/A.cat, will also result in earlyexpression of the heterologous proteins in substantially all tissues ofthe infected larvae.

Example 11 B. mori Recombinant Expression Cassettes Inserted intoAutographica californica Nuclear Polyhedrosis Virus

In this example, transplacement vectors are created which can be used togenerate recombinant Autographa californica Nuclear Polyhedrosis Viruses(AcNPVs) expressing the heterologous proteins listed in Example 10,under the control of the actin promoter of B. mori. In contrast to mostnuclear polyhedrosis viruses, AcNPV has been shown to be capable ofinfecting productively several lepidopteran families. Plasmid pAc.RI-I(Smith et al., 1983), which contains the polyhedron gene of AcNPVtogether with 4.0 kb of 5′ and 2.1 kb of 3′ flanking sequences, is used.This plasmid is linearized with EcoRV which cleaves uniquely at aposition located 40 bp upstream of the polyhedron gene, and the SstIfragments of the recombinant expression cassettes 1-8 listed in Example10 above, which contain the heterologous genes under the control of theactin promoter of B. mori, are inserted into the EcoRV site of theplasmid pAc.RI-I by blunt-end ligation to generate transplacementvectors pAcp/A.jhe, pAcp/A.enk, pAcp/A.pro, pAcp/A.aph, pAcp/A.cpr,pAcp/A.fmrf, pAcp/A.TxP-I and pAcp/A.AaIT. Previous studies (Possee andHoward, 1987) have demonstrated that insertions of foreign DNA into thissite of the genome of AcNPV leave all AcNPV functions unaffected,including that of the polyhedron gene, thus permitting the generation ofrecombinant AcNPV baculoviruses which are occluded into polyhedra.

For insertion of the recombination cassettes 1-8 on Sstl fragments intothe EcoRV site of plasmid pAc.RI-I, each plasmid containing arecombinant expression cassette as described in Example 10 is digestedwith SstI, the 3′ protruding termini generated by SstI are removed bydigestion with mung bean nuclease (New England Biolabs, Inc.), the SstIfragments containing the recombinant expression cassette are isolated byelectroelution following electrophoresis in an agarose gel, and ligatedinto the EcoRV-digested plasmid pAc.RI-I with T4 DNA ligase, asdescribed in Example 1.

Recombinant baculoviruses AcNPV/A.jhe, AcNPV/A.aph, AcNPV/A.enk,AcNPV/A.pro, AcNPV/A.cpr, AcNPV/A.fmrf, AcNPV/A.TxP-I and AcNPV/A.AaITare generated and purified in Spodoptera frugiperda (Sf21) cells usingthe DNA of the transplacement vectors above and DNA from wild-typeAcNPV, as described in Example 2.

Infection of Trichoplusia ni, B. mori or Heliothis virescens insectlarvae with such recombinant baculoviruses by the oral route isaccomplished by spreading aqueous suspensions of polyhedra prepared frominfected Sf21 cells onto the diet (foliage or artificial diets) of theseinsects and allowing the larvae to feed on the occlusion body-containingdiet. In larvae infected with these recombinant AcNPVs, expression ofthe heterologous proteins under the control of the actin promoter of B.mori will occur in essentially all insect tissues at an early stage ofintroduction.

This suggests that it will be possible to utilize expression cassettesbased on the principles described in this invention for constructingrecombinant baculoviruses to be used for the control of a variety oflepidopteran species.

Example 12 Isolation of the Enhancer element

A 1.2 kb fragment of the BmNPV genome (51.8 m.u. to 52.7 m.u.) wascloned and sequenced (FIG. 9). The 1.2 kb SspI fragment represents aportion of a larger 6.5 ClaI fragment (49.94 mu to 54.94 mu) which wasisolated following digestion of the BmNPV genome with ClaI. The 6.5 kbClaI fragment was isolated on the basis that it contains the CG30 andVP39 genes. Upon sequencing the 1.2 kb SspI fragment was shown tocontain a 559 bp sequence (boxed) similar to the hr sequences of AcNPVwhich have been shown to serve as origins of viral DNA replication andin some cases enhancers of early viral genes (Guarino et al, 1980; Kool,M., et al., 1993).

Example 13 Enhancement of Activity in Transfected Insect Cells

To determine whether or not this enhancer element could be useful inincreasing the activity of the actin promoter when used to drive theexpression of foreign genes in recombinant baculoviruses, we tested itsability to stimulate the actin-cat gene in cells infected with BmNPV.

A series of gene constructs was generated containing this putativeenhancer fragment in four different orientations relative to the B. moricytoplasmic actin promoter. The 1.2 kb SspI fragment of BmNPV DNAcontaining the hr3 sequence (FIG. 9A) was inserted in both orientationsinto the SmaI site of plasmid Bluescript SK+ (“pBS/SK+”, Stratagene) togenerate the plasmids p134 and p154. An SstI fragment from pBmA.cat(Example 1, FIG. 1B), containing the A3 cytoplasmic actin promoter of B.mori fused to the cat open reading frame, was inserted in bothorientations into an SstI site in plasmids p134 and p154 to generateplasmids p13314, p13315, p15316, and p15317 (FIG. 10A).

The ability of this DNA fragment to enhance the actin promoter wasassayed by transfecting Bm5 cells with each of the four plasmids andpBmA.cat. Bm5 cell cultures were maintained in IPL41 medium, containing10% fetal calf serum as previously described In Example 2. Bm5 cellswere transfected in 6-well microtiter plates as described in Example 2.The cells were transfected with equimolar amounts of the indicatedplasmid (either pBmA.cat, p13314, p13315, p15316 or p15317) and 1.5 μgof pBS/SK+. The cells were collected for CAT assays at 2 days aftertransfection. The CAT assays were performed as described in Example 4taking care to ensure that equal amounts of protein were tested in eachsample.

In all four configurations tested, the presence of the BmNPV enhancerDNA fragment substantially stimulated expression of the cat genefunctionally linked to the insect actin gene promoter (FIG. 10B).

Quantitation of these assays indicated that while Bm5 cells transfectedwith pBmA.cat (Example 1, FIG. 1B) contained 0.33 units CAT activity/mgprotein, cells transfected with p13315 contained 29.5 U/mg, an 88-foldstimulation (FIG. 10B).

Plasmids pBmA.cat (“A.cat”) or p13315 with pBS/SK+ were also transfectedinto Spodoptera frugiperda (Sf21) tissue culture cells by the methoddisclosed in Example 8. The 1.2 kb SspI fragment was able to stimulateexpression from the actin-cat gene in Sf21 cells by a factor of 667(FIG. 10B).

Example 14 Enhancement of Activity in Infected Insect Cells

To determine whether or not this enhancer element could be useful inincreasing the activity of the actin promoter to drive the expression offoreign genes in recombinant baculoviruses, we tested its ability tostimulate the actin-cat gene in cells infected with BmNPV. Bm5 cellswere infected with wild type BmNPV (10 pfu/cell) 4 hours prior to thetransfection with the plasmids, by the method described in Example 3.The cells were then transfected with either a mixture of 1.0 μg pBmA.catand 1.5 μg pBS/SK+ or with a mixture of 1.0 μg p13315 and 1.5 μgpBS/SK+. Cells were collected at 2 days after transfection and assayedfor CAT activity by a method similar to that described in Example 4.Three replicates of each infection/transfection are shown. In FIG. 11,(A.cat) denotes the cells receiving plasmid pBmA.cat and (eA.cat)denotes the cells receiving plasmid p13315.

The presence of the 1.2 kb hr element was able to provide a 220-foldstimulation in the amount of cat gene expression from the actin promoterin cells infected with BmNPV virus (FIG. 11A).

The above results indicate that the B. mori cytoplasmic actin promoteractivity was strongly enhanced by the presence of the 1.2 kb SspIfragment. Both the silkworm (Bm5) cells and the heterologouslepidopteran insect cells (Sf21) showed increased levels of atinactivity in the absence of the BmNPV genome. In BmNPV-infected cells thelevel of enhancement was slightly higher. These results indicate thatthe 1.2 kb baculovirus fragment can enhance the expression of an insectcellular promoter.

Example 15 Confirmation that the plasmids containing the 1.2 kb fragmentwere not present in high copy number

Because hr's have been shown to serve as origins of replication in AcNPVand can, under some conditions, allow plasmids containing thesesequences to replicate in AcNPV-infected cells (Pearson et al., 1992;Kool et al., 1993) and because the 1.2 kb SspI fragment contains asequence with homology to hr, the possibility existed that some of theincreased expression from the cat gene functionally linked to theenhancer could have been due to the presence of an increased number ofplasmids in the transfected cells. To test for this possibility, nucleiwere isolated from the cells infected with BmNPV and transfected withthe various plasmids in Example 14 and probed with labelled cat DNA.

Nuclei were isolated from the infected and transfected Bm5 cells byincubation in PBS containing 0.5% NP-40 at 0° for 10 minutes. Aftercentrifugation at 4° for 10 minutes at 2,500 rpm in a microcentrifuge,the pellet was resuspended in PBS and recentrifuged as above. Thepelleted nuclei were then suspended in PBS and counted with ahemocytometer. Nuclear suspensions were dot-blotted onto an Hybond N+membrane (Amersham Canada Ltd.) and treated with 0.5 M NaOH followed by0.5 M Tris-HCl pH 7.5. The number of nuclei blotted is indicated in FIG.11B. Labelling of a 900 bp BamHI fragment from pBmA.cat with [α-³²P]dCTP and hybridization of this probe to the membrane was carried out asdescribed in Example 8. After hybridization, the membrane was washed asdescribed in Example 8 and X-ray film was exposed to the membrane.

The plasmid containing the enhancer/actin-cat gene construct p13315(“eA.cat”) did not appear to be any more abundant in the Bm5 insectcells than the plasmid containing only the actin-cat gene constructpBmA.cat (“A.cat”) (See FIG. 11B). Thus, the greatly increased level ofCAT activity resulting from the presence of the enhancer is the resultof an actual increase in gene expression, rather than simply anamplification of the number of genes present.

Example 16 Generation of recombinant baculoviruses containing theenhancer element

To directly test the strength of the enhanced actin promoter for geneexpression in recombinant baculoviruses, an appropriate expressioncassette will be constructed. The plasmid pBmeA (FIG. 12B) is similar tothe plasmid pBmA (FIG. 12A; FIG. 1A) but additionally contains theenhancer sequence 5′ of the actin promoter.

The plasmid pBmeA is constructed by:

i. digesting plasmid pBSII-SK+ (Stratagene) with XbaI and KpnI,blunt-ending and recircularizing it with DNA ligase, thus removing allpolylinker sites located between XbaI and KpnI including the XbaI andKpnI sites to create plasmid pBSII-ΔSK+;

ii. linearizing pBSII-ΔSK+ with EagI (unique site in the polylinker ofthis plasmid), end-filling with Klenow polymerase and blunt-end ligatingthe 1.2 kb SspI enhancer fragment containing the hr3 sequence with DNAligase to generate plasmid pBSII-ΔSK+/enhancer;

iii. subcloning into the unique SstI site of plasmid pBSII-ΔSK+/enhancerthe 2.6 kb actin cassette, excised from pBmA (FIG. 12A) with SstI, togenerate the enhanced expression cassette vector pBmeA (FIG. 12B).

Any open reading frame gene can then be inserted into one of the uniquerestriction sites present in the polylinker sequence of pBmeA to placeit under the control of the enhancer-linked actin promoter.

The resultant gene fusions can then be excised as a DNA fragment withBssHII and the DNA fragment blunt-ended by end-filling with Klenowpolymerase. The blunt-ended fragment can be ligated into the PmEIrestriction site in transfer vector pBmp2p for the purpose of generatingrecombinant viruses by the method described in Example 1.

Transfer vector pBmp2p is identical to pBmp2 (Iatrou and Meidinger,1989) except that the unique XbaI cloning site of the latter has beenconverted into a PmEI site through linearization with XbaI, end fillingwith Klenow polymerase, ligation of PmEI linkers, digestion with PmEIcircularization with DNA ligase and recloning.

It is expected that recombinant viruses expressing foreign genes underthe control of the enhanced actin promoter will express at an earlystage of introduction (infection of insect cells) significantly higherquantities of foreign gene products than corresponding viruses utilizingthe basic actin promoter.

Example 17 Cloning and use of the B. mori Transactivator Gene in InsectTissue Culture Cells

A 3.8 kb fragment of the BmNPV genome containing the IE-1 gene(Huybrechts et al. (1992) was cloned into the pBS/SK+ plasmid(Stratagene) at the ClaI site to generate plasmid pBmIE1.

A 3.1 kb ClaI fragment of the AcNPV genome containing the IE-1 gene wasprovided by Dr. Jarvis, Texas A & M University. This 3.1 kb fragment wascloned into the pUC8 plasmid at the AccI site to generate plasmidpAcIE1.

The plasmids pBmIE1 or pAcIE1 were transfected into Bm5 insect cells bythe method in Example 2. At the same time the vector pBmA.cat was alsotransfected into the Bm5 cells by the same method. The level of CATexpression was measured by the method in Example 4.

It was observed that the level of expression of the CAT protein wasapproximately 100 fold greater when the transactivator gene was present(FIG. 13A). From this and other experiments (See also FIG. 14) it isapparent that the IE1 product of BmNPV or AcNPV stimulates the level ofexpression of the cellular promoter of the recombinant cellular promoterexpression cassette in trans i.e. the enhancing effect is the sameirrespective of whether the gene encoding the transactivator is linkedto the expression cassette or not.

It has further been found that the gene encoding for the trans-activatorand the trans-activator itself are both equally active in cell linesderived from other lepidopteran insects. (FIG. 13B) For example, it hasbeen found that the IE-1 gene of the AcNPV (Guarino et al (1987)) couldsubstitute for the IE-1 gene of BmNPV in both silkmoth (Bm5) andheterologous (Sf21) tissue culture cells (FIG. 13A and 13B).

By comparing cells transfected with the transactivator gene and eitherpBmA.cat or p13351 by the methods described in the Examples above, ithas been found that the enhancing effect of the trans-activator isindependent of the presence of the enhancer element.

Finally it has been found that the addition of both genetic elements tothe insect cells in conjunction with the expression cassette (i.e. theenhancer linked to the expression cassette and the gene encoding for thetrans-activator either linked to the cassette or supplied separately tothe cells in the form of the second plasmid) results in an increase ofabout 2,000 to 5,000 fold in CAT protein produced from the cat geneinserted into the recombinant expression cassette under the control ofthe cellular promoter (FIG. 14). Similar levels of enhancement in theexpression of the foreign gene product occurs also in heterologouslepidopteran cells.

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5 1189 base pairs nucleic acid double linear unknown 1 AATATTAGACAACAAAGATT TATTTTATTC ATGCCACTAC TCGGTTCCGT TTTTCAAGCT 60 AACCAGTTGTCATGCGGAAA ATGACGTCAT TATTAATGCT TTAAACGAGT TACGCAACAA 120 CGTTAAAGTGGACGCTGATT GCGATTTTTT TCAAAGACCT ATCGCACGTT TAAAACGCGT 180 ACGCTTATGTGGGCAACGGG ATTGGTTGTA GATCCGCGTA CGACGAAGAT GCGATAGTGG 240 TAAAAAAAGAAGCCGTGCCC AGTCACGTGT ACGCCAACCT GAACACGCAA TCCAACGACG 300 GCGTCAAATACAATCGTTGG TTGCACGTTA AAAACGGCCA ATACATGGCG TGTCCTGAAG 360 AATTGTACGATAACAACGAA TTTAAATGTA ACGTAGAATC GGATAAATTA TATTATTTGG 420 ATAATTTACAAGAAGATTCC GTTGTATAAA CATTTTATGA CGAAAACAAA TGACATCATT 480 CCTGATTATAATAATTTTAA TCGTGCGTTA CAAGTACAAT TCTACTTGTA AAGCGAGTTT 540 AATTTGAAAAACAAATTAGT CATTATTAAA CATGTTAACA ATCGTGTATA AAAATGACAT 600 CAGTTTAATGATGACATCAT CTCTTGATTA TGTTTTACAC GTAGAATTCT ACTCGTAAAG 660 CCGGTTCAGTTTTGAAAAAC AAATGACATC ATCTCTTGAT TATGTTTTAC ACGTAGAATT 720 CTACTCGTAAAAGCGAGTTT AGTTTTAAAA AACAAATGAC ATCATTCAGT TTTGAAAAAC 780 AAATGACATCATCTCTTGAT TGTGTTTTAC AAGTAGAATT CTACTCGTAA AGCGAGTTCA 840 GTTTTGAAAAACAAATGACC CTCTCATACA ATCGTTGAAC AATTTTAATA AATAATCTTT 900 ACAAGATTCGTTTGAAGGCC TCATAAACAA TTTATATGAT TTAATATCAA TATACTTTTT 960 CAATCTAGCCTCGAATGGGC TGTTCACAAA TTACGCTTCT TCCACAATAA TTGCGTCGTA 1020 GCAAATTGCCAAATACTTGA CGCAACTAAT AACGTCTGAA TGGGTTTCAT CTTGAGCGCA 1080 CCTCCATCATCAAAATCATA AAACGATCTA TTTGTGGGCC AAGCTGCTGT ACCGTATAAA 1140 TCGTATAATACGACGCGGAG AAATTAATTT CTGGCACGAA CGTAATATT 1189 49 base pairs nucleicacid double linear unknown 2 AGTGGTGGCC AAAGAGAAGA AGATACCTCC CCACCAAGAGACCAGAGTG 49 15 amino acids amino acid linear unknown 3 Trp Pro Lys ArgArg Arg Tyr Leu Pro Thr Lys Arg Pro Glu Trp 1 5 10 15 46 base pairsnucleic acid double linear unknown 4 AGTGGTGGCC AAAGAGAAGA TACCTCCCCACCAAGAGACC AGAGTG 46 14 amino acids amino acid linear unknown 5 Trp ProLys Arg Arg Tyr Leu Pro Thr Lys Arg Pro Glu Trp 1 5 10

What is claimed is:
 1. A method of producing heterologous protein ininsect cells comprising expressing the heterologous protein from anenhanced recombinant expression cassette such cassette comprising astructural gene encoding the heterologous protein functionally linked toan insect cellular promoter and an enhancer and the heterologous proteinis produced.
 2. The method of claim 1 wherein the enhancer comprisesthat region of the BmNPV 1.2 kb enhancer fragment which potentiatestranscription from the promoter.
 3. The method of claim 1 wherein theinsect cells further comprise a gene encoding the IE-1 product underconditions wherein the IE-1 product is produced.
 4. The method of claim3 wherein the gene encoding the IE-1 product is inserted into the genomeof the cell.
 5. The method of claim 3 wherein the gene encoding the IE-1product is present on a vector in the cell.
 6. The method of claim 5wherein the gene encoding the IE-1 product is present on the same vectoras the enhanced recombinant expression cassette.
 7. A method ofproducing heterologous protein in insect cells wherein the insect cellscomprise an IE-1 gene under conditions wherein the IE-1 product isproduced, such method comprising expressing the heterologous proteinfrom a recombinant expression cassette, such cassette comprising astructural gene encoding a protein functionally linked to an insectcellular promoter.
 8. An insect cell comprising the IE-1 gene and aninsect cellular promoter in the absence of added baculovirus.
 9. Aninsect cell according to claim 8, wherein the recombinant expressioncassette further comprises an enhancer.